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THE UNIVERSITY 
OF ILLINOIS 
LIBRARY 


Be Sy 


M8abh 


as 





Return this book on or before the 
Latest Date stamped below. A 
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Digitized by the Internet Archive 
in 2021 with funding from 
University of Illinois Urbana-Champaign 


https://archive.org/details/hereditysexOOmorg_0 





Columbia Enibersity Lectures 


HERED TY AND SEX 
THE JESUP LECTURES 


1913 


COLUMBIA 
UNIVERSITY PRESS 
SALES AGENTS 


NEW YORK: 
LEMCKE & BUECHNER 


380-32 West 27TH STREET 


LONDON: 
HUMPHREY MILFORD 


AMEN CorRNneER, E.C. 


TORONTO: 
HUMPHREY MILFORD 


25 Ricumonp 8t., W. 


COLUMBIA UNIVERSITY LECTURES 


HEREDITY AND SEX 


BY. 


BEOMAS HUNT MORGANS baad: 


PROFESSOR OF EXPERIMENTAL ZOOLOGY 
IN COLUMBIA UNIVERSITY 





Nets Work 
COLUMBIA UNIVERSITY PRESS 
1913 


All rights reserved 


CopyRiGuT, 1913, 


By COLUMBIA UNIVERSITY PRESS. 





Set up and electrotyped. Published November, 1913. 


Norwood jpress 
J. 8S. Cushing Co. — Berwick & Smith Co, 
Norwood, Mass., U.S.A, 


cv 





INTRODUCTION 


Two lines of research have developed with surpris- 
ing rapidity in recent years. Their development has 
been independent, but at many stages in their progress 
they have looked to each other for help. The study 
of the cell has furnished some fundamental facts 
connected with problems of heredity. The modern 
study of heredity has proven itself to be an instrument 
even more subtle in the analysis of the materials of 
the germ-cells than actual observations on the germ- 
cells themselves. 

In the following chapters it has been my aim to point 
out, wherever possible, the bearing of cytological 
studies on heredity, and of the study of heredity on the 
analysis of the germinal materials. 

The time has come, I think, when a failure to recog- 
nize the close bond between these two modern lines of 
advance can no longer be interpreted as a wise or 
cautious skepticism. It seems to me to indicate rather 
a failure to appreciate what is being done at present, 
and what has been accomplished. It may not be desir- 
able to accept everything that is new, but it is cer- 
tainly undesirable to reject what is new because of its 
newness, or because one has failed to keep in touch with 
the times. An anarchistic spirit in science does not 
always mean greater profundity, nor is our attitude 


toward science more correct because we are unduly 
e 


ye omy 
S4 9729 


vl INTRODUCTION 


skeptical toward every advance. Our usefulness will, 
in the long run, be proven by whether or not we have 
been discriminating and sympathetic in our attitude 
toward the important discoveries of our time. While 
every one will probably admit such generalities, some of 
us may call those who accept less than ourselves con- 
servatives; others of us who accept more will be called 
rash or intemperate. To maintain the right balance 
is the hardest task we have to meet. In attempting to 
bring together, and to interpret, work that is still in the 
making I cannot hope to have always made the right 
choice, but I may hope at least for some indulgence 
from those who realize the difficulties, and who think 
with me that it may be worth while to make the 
attempt to point out to those who are not specialists 
what specialists are thinking about and doing. 

What I most fear is that in thus attempting to for- 
mulate some of the difficult problems of present-day 
interest to zodlogists I may appear to make at times 
unqualified statements in a dogmatic spirit. I beg to 
remind the reader and possible critic that the writer 
holds all conclusions in science relative, and subject 
to change, for change in science does not mean so much 
that what has gone before was wrong as the discovery 
of a better strategic position than the one last held. 


TABLE OF CONTENTS 


INTRODUCTION 


CHAPTER I 


THE EVOLUTION OF SEX 


1. REPRODUCTION, A DISTINCTIVE FEATURE OF LIVING 
THINGS 


b 


THE “MEANING” OF SEXUAL REPRODUCTION 
THE Bopy AND THE GERM-PLASM 


THE EaAarty ISOLATION OF THE GERM-CELLS 


iets ee oS 


THE APPEARANCE OF THE ACCESSORY ORGANS OF 
REPRODUCTION 

6. THE SECONDARY SEXUAL CHARACTERS 

7. THe SEXvuAL INSTINCTS 


CHAPTER II 


THE MECHANISM OF SEX-DETERMINATION 


1. THe MATURATION OF THE EGG AND THE SPERM 


bo 


TuHE CYTOLOGICAL EVIDENCE 


a. Protenor 

b. Lygeus 

c. Oncopeltus . 

d. Ascaris : ; 
e. Aphids and Phylloxerans 


3. THE EXPERIMENTAL EVIDENCE 
a. The Experiments on Sea-urchins’ Eges 
b. The Evidence from Sex-linked Inheritance 
vii 


PAGES 


. v-vl 


1-4 
» 4-15 
15-19 
20-23 


25-26 
26-31 
31-34 


vill TABLE OF CONTENTS 


CHAPTER III 


THE MENDELIAN PRINCIPLES OF HEREDITY 
AND THEIR BEARING ON SEX 


’ PAGES 
1. MENDEL’s DISCOVERIES . : . ; 4 ; 73-715 
2. THe HEREDITY OF ONE PAIR OF CHARACTERS . ‘ 75-80 
3. THE HEREDITY OF A SEX-LINKED CHARACTER . ; 80-84 
4. Tor Herepity oF Two Parrs or CHARACTERS ; 84-88 
5. THe HerreEpitry OF Two SEX-LINKED CHARACTERS . 88-93 
6. A THEORY OF LINKAGE : : ; : ? ‘ 93-97 
7. THREE SEX-LINKED FaAcTors ; ; ; ‘ ; 98-100 
CHAP THER OLY 
SECONDARY SEXUAL CHARACTERS AND THEIR 
RELATION TO DARWIN’S THEORY OF SEX- 
UAL SELECTION 
1. THe OccuRRENCE OF SECONDARY SEXUAL CHARAC- 
TERS IN THE ANIMAL KINGDOM . : : . 101-112 
2. CouRTSHIP : ; : : : ; : ‘ . 112-120 
3. VIGOR AND SECONDARY SEXUAL CHARACTERS . . 120-121 
4, ConTINUOUS VARIATION AS A BASIS FOR SELECTION 121-125 
5. DISCONTINUOUS VARIATION OR MUTATION AS A BASIS 
FOR SELECTION : ; : : ‘ ; . 125-131 
CHAPTER V 
THE EFFECTS OF CASTRATION AND OF TRANS- 
PLANTATION ON THE SECONDARY SEXUAL 
CHARACTERS 
1. OPERATIONS ON MAMMALS . , . : ‘ . 132-141 
2. OPERATIONS ON BirpDs . : 3 , : ; . 142-144 
3. OPERATIONS ON AMPHIBIA . : é : 4 . 145-146 
4. INTERNAL SECRETIONS . ‘ : : : .  . 146-147 
5. OPERATIONS ON INSECTrs : : : . y . 148-155 
6. PARASITIC CASTRATION OF CRUSTACEA ; ; . 155-158 


TABLE OF CONTENTS 


CHAPTER VI 


1X 


GYNANDROMORPHISM, HERMAPHRODITISM, 


PARTHENOGENESIS, AND SEX 


1. GYNANDROMORPHISM 
2, HERMAPHRODITISM. 
3. PARTHENOGENESIS . 


4. ARTIFICIAL PARTHENOGENESIS 


CHAPTER VII 


FERTILITY 


1. INBREEDING 


CROSS-BREEDING 


bo 


SEXUAL REPRODUCTION IN PARAMGCIUM 


sa 


THEORIES OF FERTILITY 


CHAPTER VIII 


SPECIAL CASES OF SEX-INHERITANCE 

1. Sex in BEEs 

2. A SEX-LINKED LETHAL FACTOR 

3. NON-DISJUNCTION OF THE SEX-CHROMOSOMES 

4. Tur VANISHING MALES OF THE NEMATODES 

5. SEX-RATIOS IN HyBrip BIRDS AND IN CROSSED 
Races IN MAn 

6. SEX-RATIOS IN FROGS 

7. SEX-RATIOS IN MAN 

8. THE ABANDONED VIEW THAT EXTERNAL CONDITIONS 
DETERMINE SEX 


9. SEX-DETERMINATION: IN MAN 


BIBLIOGRHA PHY 
INDEX 


PAGE 


161-167 
167-175 
173-188 
188-195 


194-199 
200-207 
207-211 
211-219 


220-221 
221-223 


bo 
BS ob bs 
ery % 3 
bo 
teat 
en) 


nm 


5) 


2-236 
36-2 


49 


bo bo 


25ISBFS 


279-282 





HEREDITY AND SEX 


Gap ere heel 
THE EVOLUTION OF SEX 


ANIMALS and plants living to-day reproduce them- 
selves in a great variety of ways. With a modicum of 
ingenuity we can arrange the different ways in series 
beginning with the simplest and ending with the more 
complex. In a word, we can construct systems of 
evolution, and we like to think that these systems reveal 
to us something about the evolutionary process that 
has taken place. 

There can be no doubt that our minds are greatly 
impressed by the construction of a graded series of 
stages connecting the simpler with the complex. It is 
true that such a series shows us how the simple forms 
might conceivably pass by almost insensible (or at 
least by overlapping) stages to the most complicated 
forms. This evidence reassures us that a process of 
evolution could have taken place in the imagined order. 
But our satisfaction is superficial if we imagine that 
such a survey gives much insight either into the causal 
processes that have produced the successive stages, or 
into the interpretation of these stages after they have 
been produced. 

Such a series in the present case would culminate 


in a process of sexual reproduction with males and 
1 


2 HEREDITY AND SEX 


females as the acters in the drama. But if we are 
asked what advantage, if any, has resulted from 
the process of sexual reproduction, carried out on 
the two-sex scheme, we must confess to some un- 
certainty. 


The most important fact that we know about living 
matter is its inordinate power of increasing itself. If 
all the fifteen million eggs laid by the conger eel were 
to grow up, and in turn reproduce, in two years the 
sea would be a wriggling mass of fish. 

A single infusorian, produced in seven days 935 de- 
scendants. One species, stylonichia, produced in 64% 
days a mass of protoplasm weighing one’ kilogram. 
At the end of 30 days, at the same rate, the number of 
kilograms would be 1 followed by 44 zeros, or a mass of 
protoplasm a million times larger than the volume of 
the sun. 

Another minute organism, hydatina, produces about 
30 eggs. At the end of a year (65 generations), if all the 
offspring survived, they would form a sphere whose 
limits would extend beyond the confines of the known 
universe. 

The omnipresent English sparrow would produce in 
20 years, if none died except from old age, so many de- 
scendants that there would be one sparrow for every 
square inch of the State of Illinois. Even slow- 
breeding man has doubled his numbers in 25 years. 
At the same rate there would in 1000 years not 
be standing room on the surface of the earth for 
his offspring. 

I have not gone into these calculations and will 


THE EVOLUTION OF SEX 3 


not vouch for them all, but whether they are en- 
tirely correct or only partially so, they give a rough 
idea at least of the stupendous power of growth. 

There are three checks to this process: First, the 
food supply is insufficient — you starve; second, ani- 
mals eat each other — you feed; third, substances are 
produced by the activity of the body itself that inter- 
fere with its powers of growth — you poison yourself. 
The laws of food supply and the appetites of enemies 
are as inexorable as fate. Life may be defined as a 
constant attempt to find the one and avoid the other. 
But we are concerned here with the third point, the 
methods that have been devised of escape from the 
limitations of the body itself. This is found in repro- 
duction. The simplest possible device is to divide. 
This makes dispersal possible with an increased chance 
of finding food, and of escaping annihilation, and at 
the same time by reducing the mass permits of a more 
ready escape of the by-products of the living machine. 

Reproduction by simple division is a well-known pro- 
cess in many of the lower animals and plants; it is 
almost universal in one-celled forms, and not unknown 
even in many-celled organisms. Amoeba and _ para- 
moecium are the stock cases for unicellular animals; 
many plants reproduce by buds, tubers, stolons, or 
shoots; hydroids and sea-anemones both divide and 
bud; many planarians, and some worms, divide trans- 
versely to produce two new individuals. But these 
methods of reproduction are limited to simple structures 
where concentration and division of labor amongst the 
organs has not been carried to an extreme. In con- 
sequence, what each part lacks after the division can be 


4 HEREDITY AND SEX 


quickly made good, for delay, if prolonged, would 
increase the chances of death. 

But there is another method of division that is almost 
universal and is utilized by high and by low forms alike : 
individual cells, as eggs, are set free from the rest of 
the body. Since they represent so small a part of the 
body, an immense number of them may be produced on 
the chance that a few will escape the dangers of the 
long road leading to maturity. Sometimes the eggs 
are protected by jelly, or by shells, or by being trans- 
parent, or by being hidden in the ground or under 
stones, or even in the body of the parent. Under these 
circumstances the animal ventures to produce eggs with 
a large amount of food stored up for the young embryo. 

So far reaching were the benefits of reproduction 
by eggs that it has been followed by almost every 
species in the animal and plant kingdom. It is ad- 
hered to even in those cases where the animals follow 
other grosser methods of separation at the same time. 
We find, however, a strange limitation has been put 
upon the process of reproduction by eggs. Before the 
ege begins its development it must be fertilized. Cells 
from two individuals must come together to produce 
a new one. 

The meaning of this process has baffled biologists 
ever since the changes that take place during fertili- 
zation were first discovered; in fact, long before the 
actual processes that take place were in the least un- 
derstood. There is a rather extensive and antiquated 
literature dealing with the part of the male and of 
the female in the process of procreation. It would 
take us too far to attempt to deal with these questions 


THE EVOLUTION OF SEX 3) 


in their historical aspects, but some of their most 
modern aspects may well arrest our attention. 

In the simplest cases, as shown by some of the one- 
celled organisms, two individuals fuse into a single 
one (Fig. 1); in other related organisms the two in- 
dividuals that fuse may be unequal in size. Some- 
times we speak of these as male and female, but 
it is questionable whether we should apply to these 
unicellular types the same names that we use for the 





Fia. 1. — Union of two individuals (Stephanosphera pluvialis) to 
form a single individual. (After D6flein.) 


many-celled forms where the word sex applies to the 
soma or body, and not to the germ cells. 

One of the best known cases of conjugation is that 
of paramoecium. Under certain conditions two in- 
dividuals unite and partially fuse together. An in- 
terchange of certain bodies, the micronuclei, then takes 
place, as shown in Fig. 2, and in diagram, Fig. 3. The 
two conjugating paramoecia next separate, and each 
begins a new cycle of divisions. Here each individual 
may be said to have fertilized the other. The process 
recalls what takes place in hermaphroditic animals of 
higher groups in the sense that sperm from one indi- 
vidual fertilizes eggs of the other. 

We owe to Maupas the inauguration of an epoch- 
making series of studies based on phenomena like this 
in paramcecium. 


6 HEREDITY AND SEX 





Fig. 2. — Conjugation in Paramcecium. The micronucleus in one indi- 
vidual is represented in black, in the other by cross-lines. The macro- 
nucleus in both is stippled. A—C, division of micronucleus into 2 and 
4 nuclei; C'!—D, elongation of conjugation nuclei, which interchange and 
recombine in #; F—J/, consecutive stage in one ex-conjugant to show three 
divisions of new micronucleus to produce eight micronuclei (J). In lower 
part of diagram the first two divisions of the ex-conjugant (J) with eight 
micronuclei are shown, by means of which a redistribution of the eight 
micronuclei takes place, See also Fig. 100. 


“I 


THE EVOLUTION OF SEX 





Fic. 3. — The nuclei of two individuals of paramcecium in I (homozygous 
in certain factors, and heterozygous in other factors), are represented as divid- 
ing twice (in II and III); the first division, II, is represented as reducing, 
i.e. segregation occurs; the second division, III, is represented as equational, 
z.e. no reduction but division of factors, as in the next or conjugation division, 


IV, also. 


8 HEREDITY AND SEX 


Maupas found by following from generation to 
generation the division of some of these protozoa that 
the division rate slowly declines and finally comes to 
an end. He found. that if a debilitated individual 
conjugates with a wild individual, the death of the race 
is prevented, but Maupas did not claim that through 
conjugation the division rate was restored. On the 
contrary he found it is lower for a time. 

He also discovered that conjugation between two 
related individuals of these weakened strains produced 
no beneficial results. 

Biutschli had earlier (1876) suggested that conjugation 
means rejuvenation or renewal of youth, and Maupas’ 
results have sometimes been cited as supporting this 
view. Later work has thrown many doubts on this 
interpretation and has raised a number of new issues. 

In the first place, the question arose whether the 
decline that Maupas observed in the rate of division 
may not have been due to the uniform conditions under 
which his cultures were maintained, or to an insuffi- 
ciency in some ingredient of these cultures rather than 
to lack of conjugation. Probably this is true, for 
Calkins has shown that by putting a declining race 
into a different medium the original division rate may 
be restored. Woodruff has used as culture media a 
great variety of food stuffs and has succeeded in keep- 
ing his lines without loss of vigor through 3000 gen- 
erations. Maupas records a decline in other related 
protozoa at the end of a few hundred generations. 

Biitschhl’s idea that by the temporary union (with 
interchange of micronuclei) of two weak individuals 
two vigorous individuals could be produced seems 


THE EVOLUTION OF SEX 9 


mysterious; unless it can be made more explicit, it 
does not seem in accord with our physico-chemical 
conceptions. Jennings, who has more recently studied 
in greater detail the process of division and conjugation 
in paramcecium, has found evidence on which to base 
a more explicit statement as to the meaning of rejuve- 
nescence through conjugation. 

Jennings’ work is safeguarded at every turn by care- 
ful controls, and owing in large part to these controls 
his results make the interpretations more certain. He 
found in a vigorous race, that conjugated at rather 
definite intervals, that after conjugation the division 
rate was not greater than it had been before, but on 
the contrary was slower — a fact known, as he points 
out, to Maupas and to Hertwig. Conjugation does 
not rejuvenate in this sense. 

Jennings states that, since his race was at the be- 
ginning vigorous, the objection might be raised that 
the conditions were not entirely fulfilled, for his pred- 
ecessors had concluded that it is a weakened race that 
was saved from annihilation by the process. In order 
to meet this objection he took some individuals from 
his stock and reared them in a small amount of culture 
fluid on a slide. After a time they became weakened 
and their rate of division was retarded. He then al- 
lowed them to conjugate, and reared the conjugants. 
Most of these were not benefited in the least by the 
process, and soon died. A few improved and began 
to multiply, but even then not so fast as paramcecia in 
the control cultures that had been prevented from con- 
jugating. Still others gave intermediate rates of 
division. 


10 HEREDITY AND SEX 


He concludes that conjugation is not in itself bene- 
ficial to all conjugants, but that the essence of the pro- 
cess 1s that a recombination of the hereditary traits 
occurs as shown in the diagram, Fig. 3 and 4. Some 


5 ee 
ee 


‘AB | 
a 
A°D Be Aa 
Cals =e a BY 


au 
Nin S ( 
CD 


Fic. 4. — Illustrating conjugation between two stocks, with pairs of 
factors A, B, C, D, and a, b, c,d; and union of pairs into Aa, Bb, Cc, Dd. 
After these separate, their possible recombinations are shown in the 16 
smaller circles. (After Wilson.) 


of these new combinations are beneficial for special 
conditions — others not. The offspring of those con- 
jugants that have made favorable combinations will 
soon crowd out the descendants of other conjugants 
that have made mediocre or injurious combinations. 
Hence, in a mass culture containing at all times large 


THE EVOLUTION OF SEX 11 


numbers of individuals, the maximum division rate is 
kept up, because, at any one time, the majority of the 
individuals come from the combinations favorable to 
that special environment. 

There are certain points in this argument that call 
for further consideration. In a mass culture the fa- 
vorable combinations for that culture will soon be made, 
if conjugation is taking place. At least this is true if 
such combinations are homogeneous (homozygous, in 
technical language). Under such circumstances the 
race will become a pure strain, and further conjugation 
could do nothing for it even if it were transferred to a 
medium unsuited to it. 

In the ordinary division of a cell every single de- 
terminer divides and each of the new cells receives 
half of each determiner. Hence in the case of para- 
moecium all the descendants of a given paramoecium 
that are produced by division must be exactly alike. 
But in preparation for conjugation a different pro- 
cess may be supposed to take place, as in higher 
animals, among the determiners. The determiners 
unite in pairs and then, by division, separate from 
each other, Fig. 4. In consequence the number of 
determiners is reduced to half. Each group of deter- 
miners will be different from the parent group, pro- 
vided the two determiners that united were not 
identical. If after this has occurred conjugation 
takes place, the process not only restores the total 
number of determiners in each conjugant, but gives 
new groups that differ from both of the original 
groups. 

The maintenance of the equilibrium between an 


12 HEREDITY AND SEX 


organism and its environment must be a very delicate 
matter. One combination may be best suited to one 
environment, and another combination to another. 
Conjugation brings about in a population a vast num- 
ber of combinations, some of which may be suited to 
the time and place where they occur. ‘These survive 
and produce the next generation. 

Jennings’ experiments show, if I understand him 
correctly, that the race he used was not homogeneous 
in its hereditary elements; for when two individuals 
conjugated, new combinations of the elements were 
formed. It seems probable, therefore, that the chemi- 
eal equilibrium of paramcecium is maintained by the 
presence of not too much of some, or too little of other, 
hereditary materials. In a word, its favorable com- 
binations are mixed or heterozygous. 

The meaning of conjugation, and by implication, 
the meaning of fertilization in higher forms is from this 
point of view as follows :— In many forms the race, as a 
whole, is best maintained by adapting itself to a widely 
varied environment. <A heterozygous or hybrid con- 
stitution makes this possible, and is more likely to 
perpetuate itself in the long run than a homozygous 
race that is from the nature of the case suited to a more 
limited range of external conditions. 

What bearing has this conclusion on the problem of 
the evolution of sex and of sexual reproduction ? 

This is a question that is certain to be asked. I am 
not sure that it is wise to try to answer it at present, 
in the first place because of the uncertainty about the 
conclusions themselves, and in the next place, because, 
personally, I think it very unfair and often very unfor- 


THE EVOLUTION OF SEX 13 


tunate to measure the importance of every result by 
its relation to the theory of evolution. But with this 
understanding I may venture upon a few suggestions. 

If a variation should arise in a _ hermaphroditic 
species (already reproducing sexually) that made cross- 
fertilization more likely than self-fertilization, and if, 
as a rule, the hybrid condition (however this may be 
explained) is more vigorous in the sense that it leaves 
more offspring, such a variation would survive, other 
things being equal. 

But the establishment of the contrivance in the 
species by means of which it is more likely to cross- 
fertilize, might in another sense act as a drawback. 
Should weak individuals appear, they, too, may be 
perpetuated, for on crossing, their weakness is concealed 
and their offspring are vigorous owing to their hybrid 
condition. The race will be the loser in so far as re- 
cessive or weak combinations will continue to appear, 
as they do in many small communities that have some 
deficiency in their race ; but it is a question whether the 
vigor that comes from mixing may not more than com- 
pensate for the loss due to the continual appearance of 
weakened individuals. 

This argument applies to a supposed advantage 
within the species. But recombination of what already 
exists will not lead to the development of anything 
that is essentially new. Evolution, however, is con- 
cerned with the appearance and maintenance of new 
characters. Admitting that sexual reproduction proved 
an advantage to species, and especially so when com- 
bined with a better chance of cross-fertilization, the 
machinery would be at hand by means of which any 


14 HEREDITY AND SEX 


’ 


new character that appeared would be grafted, so to 
speak, on to the body of the species in which it appeared. 
Once introduced it would be brought into combination 
with all the possible combinations, or races, already 
existing within the species. Some of the hybrid com- 
binations thus formed might be very vigorous and would 
survive. This reasoning, while hypothetical, and, per- 
haps not convincing, points at least to a way in 
which new varieties may become incorporated into 
the body of a species and assist in the process of 
evolution. 

It might be argued against this view that:the same 
end would be gained, if a new advantageous variation 
arose in a species that propagated by non-sexual 
methods or in a species that propagated by self-fertili- 
zation. The offspring of such individuals would trans- - 
mit their new character more directly to the offspring. 
Evolution may, of course, at times have come about 
in this way, and it is known that in many plants self- 
fertilization is largely or exclusively followed. But in 
a species in which cross-fertilization was the estab- 
lished means of propagation, the new character would 
be brought into relation with all the other variations 
that are found in the component races and increase 
thereby its chances of favorable combinations. We 
have in recent years come to see that a new heritable 
character is not lost by crossing, or even weakened by 
‘blending,’ as was formerly supposed to be the case ; 
hence no loss to the character itself will result in the 
union with other strains, or races, within the species. 

If then we cannot explain the origin of sexual re- 
production by means of the theory of evolution, we 


THE EVOLUTION OF SEX 15 


can at least see how the process once begun might be 
utilized in the building up of new combinations; and 
to-day evolution has come to mean not so much a 
study of the origination of new characters as the method 
by which new characters become established after they 
have appeared. 


THE BODY AND THE GERM-PLASM 


As I have said, it is not unusual to speak of the uni- 
cellular animals and plants as sexual individuals, and 
where one of them is larger than the other it is some- 
times called the female and the smaller the male. But 
in many-celled animals we mean by sex something 
different, for the term applies to the body or soma, and 
not to the reproductive cells at all. The reproductive 
cells are eggs and sperm. It leads to a good deal of 
confusion to speak of the reproductive cells as male 
and female. In the next chapter it will be pointed out 
that the eggs and sperm carry certain materials; and 
that certain combinations of these materials, after fer- 
tilization has occurred, produce females; other combi- 
nations produce males ; but males and females, as such, 
do not exist until after fertilization has taken place. 

The first step, then, in the evolution of sex was taken 
when colonies of many cells appeared. We find a 
division of labor in these many-celled organisms; the 
germ-cells are hidden away inside and are kept apart 
from the wear and tear of lfe. Their maintenance 
and protection are taken over by the other cells of the 
colony. Even among the simplest colonial forms we 
find that some colonies become specialized for the pro- 
duction of small, active germ-cells. These colonies 


16 HEREDITY AND SEX 


are called the males, or sperm-producing colonies. The 
other colonies specialize to produce larger germ-cells — 
the eggs. These colonies are called females or egg-pro- 
ducing colonies. Sex has appeared in the living world. 

To-day we are only beginning to appreciate the far- 
reaching significance of this separation into the immor- 
tal germ-cells and the mortal body, for there emerges 
the possibility of endless relations between the body on 
the one hand and the germ-cells on the other. What- 
ever the body shows in the way of new characters 
or new ways of reacting must somehow be represented 
in the germ-cells if such characters are to be perpetu- 
ated. The germ-cells show no visible modification to 
represent their potential characters. Hence the classi- 
cal conundrum — whether the hen appeared before the 
egg, or the egg before the hen? Modern biology has 
answered the question with some assurance. The egg 
came first, the hen afterwards, we answer dogmati- 
cally, because we can understand how any change in 
the egg will show itself in the next generation — in 
the new hen, for instance; but despite a vast amount 
of arguing no one has shown how a new hen could get 
her newness into the old-fashioned eggs. 

Few biological questions have been more combated 
than this attempt to isolate the germ-tract from the 
influence of the body. Nussbaum was amongst the 
first, if not the first, to draw attention to this distine- 
tion, but the credit of pointing out its importance is 
generally given.to Weismann, whose fascinating specu- 
lations start from this idea. For Weismann, the germ- 
cells are immortal — the soma alone has the stigma of 
death upon it. Each generation hands to the next 


THE EVOLUTION OF SEX 17 


one the immortal stream unmodified by the experience 
of the body. What we call the individual, male or 
female, is the protecting husk. In a sense the body is 
transient — temporary. Its chief ‘‘purpose’’ is not 
its individual life, so much as its power to support and 
carry to the next point the all important reproductive 
material. 

Modern research has gone far towards establishing 
Weismann’s claims in this regard. It is true that the 
germ-plasm must sometimes change — otherwise there 
could be no evolution. But the evidence that the germ- 
plasm responds directly to the experiences of the body 
has no substantial evidence in its support. I know, of 
course, that the whole Lamarckian school rests its 
argument on the assumption that the germ-plasm re- 
sponds to all profound changes in the soma; but despite 
the very large literature that has grown up dealing with 
this matter, proof is still lacking. And there is abun- 
dant evidence to the contrary. 

On the other hand, there is evidence to show that 
the germ-plasm does sometimes change or is changed. 
Weismann’s attempt to refer all such changes to recom- 
binations of internal factors in the germ-plasm_ it- 
self has not met with much success. Admitting that 
new combinations may be brought about in_ this 
way, as explained for paramcecium, yet it seems un- 
likely that the entire process of evolution could have 
resulted by recombining what already existed; for 
it would mean, if taken at its face value, that by re- 
combination of the differences already present in the 
first living material, all of the higher animals and plants 
were foreordained. In some way, therefore, the germ- 


18 HEREDITY AND SEX 


plasm must have changed. We have then the alter- 
natives. Is there some internal, initial or driving im- 
pulse that has led to the process of evolution? Orhas 
the environment brought about changes in the germ- 
plasm? We can only reply that the assumption of an 


e } 


a 
(tis 
4 
” 
.* Sr AE 
ead Py if 
= eZ, » ae 
fom, Sox? 
\ Se : s 
ff 
(i, ] 
US | 
as \ . « ‘ Sg 
2) We 
r 
Fig. 5.— Schematic representation of the processes occurring during 


the fertilization and subsequent segmentation of the ovum. (Boveri, from 
Howell.) 


internal force puts the problem beyond the field of 
scientific explanation. On the other hand, there is a 
small amount of evidence, very incomplete and in- 
sufficient at present, to show that changes in the en- 
vironment reach through the soma and modify the 
germinal material. 


THE EVOLUTION OF SEX 19 


It would take us too far from our immediate subject 
to attempt to discuss this matter, but it has been nec- 
essary to refer to it in passing, for it lies at the founda- 
tion of all questions of heredity and even involves, as 
we shall see later, the question of heredity of sex, 

This brings us back once more to the provisional 
conclusion we reached in connection with the experi- 
ments on paramoecium. When the egg is fertilized 
by the sperm, Fig. 5, the result is essentially the same 
as that which takes place when two parameecia fer- 
tilize each other. The sperm brings into the egg a 
nucleus that combines with the egg-nucleus. The new 
individual is formed by recombining the hereditary 
traits of its two parents. 

It is evident that fertilization accomplishes the same 
result as conjugation. If our conclusion for paramoe- 
cium holds we can understand how animals and plants 
with eggs and sperm may better readjust themselves 
now to this, now to that environment, within certain 
limits. But we cannot conclude, as I have said, that this 
process can make any permanent contribution to evolu- 
tion. It is true that Weismann has advanced the hy- 
pothesis that such recombinations furnish the materials 
for evolution, but as I have said there is no evidence 
that supports or even makes plausible his contention. 

I bring up again this point to emphasize that while the 
conclusion we arrived at — a provisional conclusion at 
best — may help us to understand how sexual repro- 
duction might be beneficial to a species in maintaining 
itself, it cannot be utilized to explain the progressive 
advances that we must believe to have taken place 
during evolution. | 


20 HEREDITY AND SEX 


THE EARLY ISOLATION OF THE GERM-CELLS 


There is much evidence to show that the germ-cells 
appear very early in the development of the individual 
when they are set aside from the cells that differentiate 
into the body cells. This need not mean that the germ- 
cells have remained unmodified, although this is at 









——_> 


Wee V 

















\\ 





Fic. 6. — Chromatin diminution-and origin of the germ-cells in Ascaris. 
(After Boveri.) 


THE EVOLUTION OF SEX 21 


first sight the most natural interpretation. It might be 
said, indeed, that they are among the first cells to 
differentiate, but only in the sense that they specialize, 
as germ-cells. 





Fig. 7. — Origin of germ-cells in Sagitta. (From 
Korschelt and Heider.) 

In a parasitic worm, ascaris, one of the first four 
cells divides differently from the other three cells. As 
seen in Fig. 6, this cell retains at its division all of its 
chromatin material, while in the other three cells some 
of the chromatin is thrown out into the cell-plasm. The 





Fie. 8. — Origin of germ-cells in Miastor. Note small black proto- 
plasmic area at bottom of egg into which one of the migrating segmentation 
nuclei moves to produce the germ-cells. (After Kahle.) 


22 HEREDITY AND SEX 


single cell that retains all of the chromatin in its nucleus 
gives rise to the germ-cells. 

In a marine worm-like form, sagitta, two cells can 
easily be distinguished from the other cells in the wall of 
the digestive tract (Fig. 7). They leave their first posi- 
tion and move into the interior of the body, where they 
produce the ovary and testes. 


Chrysemys 


EX 
CEN 

RY sR ane 

Y 





Fic. 9. — Origin of germ-cells in certain vertebrates, viz. turtle, frog, 
gar-pike and bow-fin. The germ-cells as darker cells are seen migrating from 
the digestive tract (endoderm). (After Allen.) 


In several of the insects it has been shown that at a 
very early stage in the segmentation, one, or a few cells 
at most, lying at one end of the egg develop almost in- 
dependently of the rest of the embryo (Fig. 8). Later 
they are drawn into the interior, and take up their 
final location, where they give rise to the germ-cells. 

Even in the vertebrates, where, according to the 


THE EVOLUTION OF SEX 23 


earlier accounts, the germ-cells were described as appear- 
ing late in embryonic development, it has been shown 
that the germ-cells can be detected at a very early stage 
in the walls of the digestive tract (Fig.9).. Thence they 
migrate to their definitive position, and give rise to 
the cells from which the eggs or the sperm arise. 

The germ-cells are in fact often the earliest cells to 
specialize in the sense that they are set aside from the 
other cells that produce the soma or body of the in- 
dividual. 


THE APPEARANCE OF .THE ACCESSORY ORGANS OF 
REPRODUCTION 


As animals became larger the problem of setting free 
the germ-cells was a matter of great importance. Sys- 
tems of outlets arose — the organism became piped, as it 
were. In the lower animals the germ-cells are brought 
to the surface and set free directly, and fertilization is a 
question of the chance meeting of sperm and egg; for 
there is practically no evidence to show that the sperm 
is attracted to the egg and much evidence that it is 
not. Later, the copulatory organs were evolved in all 
the higher groups of animals by means of which the 
sperm of the male is transferred directly to the female. 
This makes more certain the fertilization of the egg. 

In the mollusks, in the insects and crustaceans, and 
in the vertebrates the organs of copulation serve to 
hold the individuals together during the act of mating, 
and at the same time serve to transfer the semen of the 
male to the oviduct, or to special receptacles of the 
female. Highly elaborated systems of organs and 
special instincts, no less elaborate, serve to make the 





24 HEREDITY AND SEX 


union possible. In some types mating must occur for 
each output of eggs, but in other cases the sperm is 
stored up in special receptacles connected with the ducts 
of the female. From these receptacles a few sperm at 
a time may be set free to fertilize each egg as it passes 
the opening of the receptaculum. In the queen bee 
enough sperm is stored up to last the queen for five or 
six years and enough to fertilize a million eggs. 





Fia. 10.— Squid: Two upper right-hand figures illustrate two methods 
of copulation. Lower right-hand figure dissected to show spermatophore 
placed in mantle cavity of female. Left-hand figure (below), spermatophore 
pocket behind mouth of male; upper figure, section of same. (After Drew.) 


There are a few cases where the transfer from the 
male to the female is brought about in a different way. 
The most striking cases are those of the squids and 
octopi, and of the spiders. 

In the squid, the male and female interlock arms 
(Fig. 10). The male takes the packets of sperm (that 
are emitted at this time from the sperm-duct) by means 
of a special arm, and transfers the packets either to a 


THE EVOLUTION OF SEX 25 


special receptacle within the circle of arms of the female, 
or plants them within the mantle chamber itself of the 
female. Each packet of spermatozoa is contained in a 
long tube. On coming in contact with sea water the 
tube everts at one end, and allows the sperm to escape. 





Fic. 11.— Octopus, male showing hectocotyl arm (ha). Cop- 
ulation (below), small male, A; large female, B. 


After separation the female deposits her strings of 
eggs, which are fertilized by the sperm escaping from 
the spermatophores. In octopus and its allies, one 
arm, that is used to transfer the spermatophores, is 
specially modified at the breeding season (Fig. 11). 


26 HEREDITY AND SEX 


This arm is inserted by the male, as shown in the figure, 
within the mantle chamber of the female. In some 
species, Argonauta argo for instance (Fig. 12), the arm 





Fig. 12.— Argonauta showing developing (A) and developed (B) 
hectocotyl arm, which, after being charged with spermatophores, is left in 
mantle of female. 


is broken off, and remains attached by its suckers in- 
side the mantle of the female. The eggs are later fer- 
tilized by sperm set free from this ‘‘hectocotylized”’ arm. 


THE SECONDARY SEXUAL CHARACTERS 


In the most highly evolved stages in the evolution 
of sex a new kind of character makes its appearance. 
This is the secondary sexual character. In most cases 
such characters are more elaborate in the male, but 
occasionally in the female. They are the most aston- 
ishing thing that nature has done: brilliant colors, 
plumes, combs, wattles, and spurs, scent glands (pleas- 
ant and unpleasant); red spots, yellow spots, green 
spots, topknots and tails, horns, lanterns for the dark, 
songs, howlings, dances and tourneys — a medley of 
odds and ends. 

The most familiar examples of these characters are 
found in vertebrates and insects, while in lower forms 


THE EVOLUTION OF SEX 27 


they are rare or absent altogether. In mammals the 
horns of the male stag are excellent examples of second- 
ary sexual characters. The male sea cow is much 
greater in size than the female, and possesses long tusks. 
The mane of the lion is absent in the lioness. 





Fig. 13.— Great bird of Paradise, male and female. 
(After Elliot.) 


In birds there are many cases in which the sexes differ 
in color (Figs. 13 and 14). The male is often more 
brilliantly colored than the female and in other cases 
the nuptial plumage of the male is quite different from 
the plumage of the female. For example, the black 
and yellow colors of the male bobolink are in striking 
contrast with the brown-streaked female (Fig. 15). 
The male scarlet tanager has a fiery red plumage with 
black wings, while the female is olive green. The male 


28 HEREDITY AND SEX 


of the mallard duck has a green head and a reddish 
breast (Fig. 16), while the female is streaked with brown. 

In insects the males of some species of beetles have 
horns on the head that are lacking in the female (Fig. 
17). The males of many species of butterflies are col- 
ored differently from the females. 





Fic. 14. — White-booted humming bird, two males 
and one female. (After Gould.) 


The phosphorescent organ of our common firefly, 
Photinus pyralis, is a beautiful illustration of a second- 
ary sexual character. On the under surface of the male 
there are two bands and of the female there is a single 
band that can be illuminated (Fig. 18). At night the 
males leave their concealment and fly about. <A little 
later the females ascend to the tops of blades of grass 


a 


THE EVOLUTION OF SEX 





Fic. 15. — Male and female bobolink. 
-sbird Lore. *) 





Fic. 16. — Male and female mallard duck. 
‘“‘ Bird Lore.’’) 


(From 


(From 


29 


30 | HEREDITY AND SEX 


and remain there without glowing. A male passes by 
and flashes his light; the female flashes back. In- 
stantly he turns in his course to the spot whence the 
signal came and alights. He signals again. She re- 
plies. He ascends the blade, and if he cannot find her, 
he signals again and she responds. The signals con- 





Fic. 17. — Male and female Hercules 
beetle. (After Kingsley.) 


tinue until the female is found, and the drama of sex 
is finished. 

Mast has recently shown that the female firefly does 
more than simply respond to the signal of the male. 
If a male flies above and to the right of the female, she 
bends her abdomen so that its ventral surface is turned 
upward and to the right. If the male is above and to 
the left, the light is turned in this direction. If the male 


THE EVOLUTION OF SEX oLyK 


is directly above, the abdomen of the female is twisted 
almost upward. Butif the male is below her, she emits 
her light without turning the body. In the firefly the 
evidence that the phosphorescent organ is of use in 
bringing the sexes together seems well established. 





Fig. 18. — Male and female firefly. 


Whether all secondary sexual organs are useful in 
mating is a question that must be referred to a later 
chapter. 


THE SEXUAL INSTINCTS 


Side by side with the evolution of these many kinds 
of structural difference the sexual instincts have evolved. 
It is only in the lowest forms that the meeting of the 
egg and sperm is left to chance. The instincts that 
bring the males and females together at the mating 
season, the behavior of the individuals at this time in 


32 HEREDITY AND SEX 


relation to each other, forms one of the most curious 
chapters in the evolution of sex, for it involves court- 
ship between the males and females; the pairing or 
union of the sexes and subsequently the building of 
the nest, the care, the protection and feeding of the 
young, by one or both parents. The origin of these 
types of behavior is part of the process of evolution of 
sex; the manner of their transmission in heredity and 
their segregation according to sex is one of the most 
difficult questions in heredity — one about which noth- 
ing was known until within recent years, when a 
beginning at least has been made. 

A few samples taken almost at random will illustrate 
some of the familiar features in the psychology of sex. 
Birds have evolved some of the most complicated types 
of courtship that are known. It is in this group, too, 
as we have seen, that the development of secondary 
sexual characters has reached perhaps its highest types. 
But it is not necessarily in the species that have the 
most striking differences between the sexes that the 
courtship is most elaborate. In pigeons and their 
allies, for example, the courtship is prolonged and elab- 
orate, yet the males and females are externally al- 
most indistinguishable ; while in the barnyard fowl and 
in ducks the process is relatively simple, yet chanti- 
cleer is notoriously overdressed. 

Even in forms so simply organized as the fishes it is 
known that the sexual instincts are well developed. 
In the common minnow, fundulus, the males develop in 
the breeding season elaborate systems of tactile organs. 
The male swims by the side of the female, pressing 
his body against her side, which causes her to set free 


THE EVOLUTION OF SEX 30 


a few eggs. At the same time the male sets free the 
sperm, thereby increasing the chance that some of the 
spermatozoa will reach the egg. 

In bees, the sexual life of the hive is highly special- 
ized. Mating never occurs in the hive, but when the 
young queen takes her nuptial flight she is followed by 
the drones that up to this have led an indolent and use- 
less life in the colony. Mating occurs high in the air. 
The queen goes to the new nest and is followed by a 
swarm of workers who construct for her a new home. 
Here she remains for the rest of her life, fed and cared 
for by the workers, who give her the most assiduous 
attention — an attention that might be compared to 
courting were it not that the workers are not males 
but only immature females. The occurrence of these 
instincts in the workers that never leave or rarely at 
least leave offspring of their own is a special field of 
heredity about which we can do little more than specu- 
late. This much, however, may be hazarded. The 
inheritance of the queen and of the worker is the same. 
We know from experimental evidence that the amount 
of food given to the young grub, when it hatches from 
the egg, is the external agent that makes the grub a 
queen or a worker. In the worker the sex glands are 
little developed. Possibly their failure to develop may 
in part account for the different behavior of the workers 
and of the queen. I shall devote a special chapter to 
this question of the influence of the secretions of the 
sex glands or reproductive organs on the character of 
the body. We shall see that in some animals at least 
an important relation exists between them. 

In the spiders the mating presents a strange spectacle. 


34 HEREDITY AND SEX 


Let us follow Montgomery’s careful observations on 
Phidippus purpuratus. The male spun a small web 
of threads from the floor to one side of his cage at an 
angle of 45°. ‘‘Four minutes later he deposited a 
minute drop of sperm on it, barely visible to the naked 
eye; then extending his body over the web reached his 
palpi downwards and backwards, applying them al- 
ternately. against the drop; the palpal organs were 
pressed, not against the free surface of the drop, but 
against the other side of the web.’ Later, a minute 
drop of sperm is found sticking to the apex of one of the 
palpi. In 1678 Lister had shown that the male applies 
his palpi to the genital aperture of the female; but not 
until 1843 was it found by Menge that the palpi carry 
the sperm drop. 

In man, courtship may be an involved affair. Much 
of our literature revolves about this period, while paint- 
ing and sculpture take physical beauty as their theme. 
Unsatiated with the natural differences that distinguish 
the sexes, man adds personal adornment which reaches 
its climax in the period of courtship, and leaves a 
lasting impression on the costuming of the sexes. 
Nowhere in the animal kingdom do we find such a 
mighty display; and clothes as ornaments excel the 
most elaborate developments of secondary sexual char- 
acters of creatures lower in the scale. 

I have sketched in briefest outline some of the gen- 
eral and more familiar aspects of sex and the evolution 
of the sexes. In the chapters that follow we shall take 
up in greater detail many of the problems that have 
been only touched upon here. 


CORA PARAS GL 
THE MECHANISM OF SEX-DETERMINATION 


IN many species of animals and plants two kinds of 
individuals are produced in every generation. This 
process occurs with such regularity and persistence that 
our minds naturally seek some mechanism, some sort 
of orderly machinery, by which this condition is brought 
about. Yet from the time of Aristotle almost to the 
present day the problem has baffled completely all 
attempts at its solution. However, the solution is very 
simple. Now that we hold the situation -in our grasp, 
it seems surprising that no one was keen enough to 
deduce it by purely theoretical reasoning. At least 
the general principles involved might have been de- 
duced, although we can see that without an intimate 
knowledge of the changes that take place in the germ- 
cells the actual mechanism could never have been 
foretold. 

The bodies of animals and plants are composed of 
millions of protoplasm-filled compartments that are 
called cells. In the middle of each cell there is a sphere, 
or nucleus, containing filaments called chromosomes 
(Fig. 5). 

At each division of a cell the wall of the nucleus is 
absorbed, and the thread-like chromosomes contract. 
into rod-shaped, or rounded bodies (Fig. 6). Each 
chromosome splits lengthwise into halves; the halves 

30 


36 HEREDITY AND SEX 


are brought into relation with a spindle-shaped system 
of lines, and move apart along these lines to opposite 
sides of the cell. The protoplasm of the cell next con- 
stricts to produce two daughter cells, each containing 
a group of daughter chromosomes. 





el Drs 






a 


Te 
e 


Meacnas, 
es. 
e 


en. 
@ 


od) @ 


Seer 


ee 
¢@ 


ha 


Fic. 19. — Fertilization and polar-body formation of Nereis. The 
four smaller figures show entrance of sperm. The extrusion of the first 
polar body is shown in lower left-hand figure and of the second polar body 
in the two large right-hand figures. The last three also show the formation 
of the sperm asters, which is the beginning of the first cleavage spindle in 
the egg. (After F. R. Lillie.) 


The egg is also a cell, and in its earlier stages contains 
the same number of chromosomes as do the other cells of 
the body; but after two peculiar divisions that take 
place at maturation the number of the chromosomes is 
reduced to half. 


THE MECHANISM OF SEX-DETERMINATION 37 


( But before this time the egg-cells divide, like all the 
other cells of the body. In this way a large number 
of eggs is produced.) After a time they cease to divide 
and begin to grow larger, laying up yolk and other 
materials. At this time, the chromosomes unite in 
pairs, so that their number seems to be reduced to half 
the original number. At the final stage in the matura- 
tion of the egg, two peculiar divisions take place that 
involve the formation of two minute cells given off at 
one pole — the polar bodies. In some eggs, as in the 
sea urchin, the polar bodies are given off while the egg 
is still in the ovary and before fertilization; in other 
eggs, as in the frog, one polar body is given off before 
fertilization, the other after the sperm has entered ; 
and in other eggs, as in nereis (Fig. 19), both polar 
bodies are given off after fertilization. 

The formation of the polar bodies is a true cell- 
division, but one that is unique in two respects. 
First, one of the cells is extremely small, as seen in 
Fig. 19. The smallness is due to the minute amount 
of protoplasm that it contains. Second, the number of 
chromosomes at each division is the half or “ haploid ”’ 
number. There is much evidence to show that at one 
or at the other of these two divisions the two chromo- 
somes that had earlier united are separated, and in this 
respect this division differs from all other cell-divisions. 
In consequence, the egg nucleus, that re-forms after the 
second polar body has been produced, contains only 
half the actual number of chromosomes characteristic 
of all the other cells of the female. 

In the formation of the spermatozoa a process takes 
place almost identical with the process just described 


38 HEREDITY AND SEX 


for the female (Fig. 20). In their earlier history the 
germ-cells of the male divide with the full number of 
chromosomes characteristic of the male, which may be 
one less chromosome than in the female. The early 





F G 

Fia, 20. — A-—B, somatic cell division with four chromosomes, C-H, 

the two maturation divisions to produce the four cells (A) that become 
spermatozoa. (After Wilson.) , 


germ-cells then cease to divide for a time, and begin 
to grow, laying up yolk and other materials. At this 
time the chromosomes unite in pairs, so that the num- 
ber appears to be reduced to half. Later two divisions 
occur (Fig. 20, D-H), in one of which the united chro- 
mosomes separate. The male germ-cells differ, how- 


THE MECHANISM OF SEX-DETERMINATION 39 


ever, from the female, in that at each of these two di- 
visions the cells are equal in size. Thus four sperm- 
cells are produced from each original cell, all four pro- 
duce tails, and become spermatozoa. 

At the time of fertilization, when the spermatozo6n 
touches the surface of the egg, the egg pushes out a cone 
of protoplasm at the point of contact (Fig. 19), and, 
lending a helping hand, as it were, to the sperm, draws 
it into the egg. The projecting cone of protoplasm 
is called the fertilization cone. In a few minutes the 
head of the sperm has entered. Its tail is often left 
outside. The head absorbs fluid from the egg and 
becomes the sperm nucleus, which passes towards the 
center of the egg. Here it comes to lie by the side of the 
egg nucleus, and the two fuse. The walls of the com- 
bined nuclei dissolve away and the chromosomes appear. 
Half of these are derived from the father through 
the nucleus of the spérm, and half from the mother 
through the egg nucleus. If we count the paternal 
chromosomes, there are half as many of them as there 
are chromosomes in each cell of the body of the father. 
Presently I shall point out that this statement is not 
always true, and on this little fact, that it 1s not quite 
true, hangs the whole story of sex-determination. 

What is the meaning of these curious changes that 
have taken place in the egg and sperm? Why has the 
egg deliberately, as it were, twice thrown away its most 
. valuable heritage — its chromatin material? We do 
not know with certainty, but one consequence at least 
stands out clearly! Before the egg gave off its polar 
bodies it had the full, or diploid, number of chromo- 
somes. After this event it has only half as many. A 


40 HEREDITY AND SEX 


similar reduction occurs in the sperm, excepting that no 
chromatin is lost, but is redistributed amongst four 
spermatozoa. Egg and sperm-nucleus each have in 
consequence the haploid or half number. By combin- 
ing they bring up the number to that characteristic of 
the species. 

The history of the germ-cells, that we have just 
traced, is the background of our knowledge of the pro- 
cess of heredity in so far as observable changes in the 
germ-cells have been made out. Weowe to Weismann 
more than to any other biologist the realization of 
the importance of these changes. It is true that 
Weismann contributed only a part of the actual facts 
on which the interpretation rests. Many workers, 
and a few leaders, have laboriously made out the com- 
plete account. But Weismann, by pointing out the 
supreme importance of the changes that take place at 
this time, has furnished a stimulus that has acted like 
yeast in the minds of less imaginative workers. 

We are now in a position to apply this knowledge to 
the interpretation of the mechanism by means of which 
sex 1s determined. 


THE CYTOLOGICAL EVIDENCE 


If we study by means of modern histological methods 
the body cells of the male of the insect, Protenor 
belfragei, we find, when each cell is about to divide, 
that a group of chromosomes appears like that shown 
in Fig.21, A. There are twelve ordinary oval chromo- 
somes, and one much larger than the rest. This group 
of chromosomes is characteristic of all divisions of the 
cells of the body, regardless of whether the cells belong 


THE MECHANISM OF SEX-DETERMINATION 41 


to muscle, skin, gland, ganglion, or connective tissue. 
The early germ-cells of the male, the so-called ‘‘sper- 
matogonia,” also have thissame number. It is not until 
a later stage in their development that a remarkable 
change takes place in them. When this change occurs 
the thread-like chromosomes unite in pairs. This is 
the synapsis stage — the word means to fuse together. 

It is the most difficult stage to interpret in the whole 
history of the germ-cells. In a few forms where the 
changes that take place have been seen to best advan- 
tage it is found that chromosomes are in the form of 
long threads and that these threads unite in pairs to 
make thicker threads. When the process is completed, 
we find half as many threads as there were before. This 
statement is not quite true. In the case of the 
male protenor, for instance, there are twelve ordinary 
chromosomes and one large one. The twelve unite in 
pairs at synapsis, so that there are six double chromo- 
somes, but the large one has no mate (Fig. 21, B). 
When the others have united in synapsis, it has taken 
no part in the process, hence the reduced number of 
chromosomes in the male is seven — the seventh is 
the sex chromosome. 

‘Two divisions now follow each other in rapid succes- 
sion (Fig. 21, C, D). In the first division (C) each 
chromosome divides — seven go to one pole and seven 
to the other pole. Two cells, the primary spermato- 
cytes, are produced. Without resting, another divi- 
sion takes place (D) in each of these two cells. It is 
the second spermatocyte division. Each of the six 
ordinary chromosomes divides, but the large sex chro- 
mosome does not divide, and, lagging behind the others, 


42 HEREDITY AND SEX 


as shown in the figure (D), it passes to one pole. Hach 
secondary spermatocyte produces, therefore, two cells — 
one with six, the other with seven chromosomes. These 
cells become spermatozoa (HH’), the ones with seven 
chromosomes are the female-producing spermatozoa, the 
ones with six chromosomes are the male-producing 


Proteror 


























eee 
6 — ih 
eae? SG ih th > z 
na 
Ny | 


spermatozoa. These two classes of spermatozoa are 
present in equal numbers. 

If we study the body cells of the female protenor, we 
find fourteen chromosomes (Fig. 22, A). Twelve of 
these are the ordinary chromosomes, and two, larger 
than the rest, are the sex chromosomes. At the synap- 
sis stage all of the chromosomes unite in pairs, including 
the two sex chromosomes. When the process is finished, 
there are seven double chromosomes (Fig. 22, B). 


THE MECHANISM OF SEX-DETERMINATION 43 


When the egg sends off its two polar bodies, the chro- 
mosomes divide or separate. At the first division seven 
chromosomes pass out (C), and seven remain in the 
egg. At the next division the seven chrcemosomes in 
the egg divide again, seven pass out and seven remain 


Prolerior a) 





HrGs e228 = 


in the egg (D). Of these seven, one chromosome, 
recognizable by its large size, is the sex chromosome. 

All the eggs are alike (#). There is only one kind of 
egg, but there are two kinds of sperm. Any egg that 
is fertilized by a sperm carrying six chromosomes pro- 
duces an individual with thirteen chromosomes. This 
individual is a male. 

Any egg that is fertilized by a sperm carrying seven 


4 HEREDITY AND SEX 


chromosomes produces an individual with fourteen 
chromosomes. ‘This individual is a female. 

In another species of insect, Lygzeus bicrucis, the male 
differs from the female, not in having a different 


Lygaeus OC 


@ o®@ YIN i 
& \ 
@ 40808 Ai. 
2@ 
Se @@o e 
rs 





\/ 
D 





ae W INQ 





o* \ Hl 
‘ts : | mh *< GS 


r aw Hi A, ae 


iy 
@e0°800 


Ye 


number of chromosomes as in protenor, but by the 
occurrence of a pair of different-sized chromosomes. 
The body cells of the male have twelve ordinary 
chromosomes and two sex chromosomes — one larger, 
xX than: thesotherwy 3 hie.23) Ag: 
After synapsis there are six double chromosomes and 
the two sex chromosomes, called X and Y (Fig. 23, D). 


BIGe 223% 


THE MECHANISM OF SEX-DETERMINATION 45 


At the first spermatocyte division all the chromosomes 
divide (C). The two resulting cells have eight chro- 
mosomes, including X and Y. At the second division 
(D) the double chromosomes again divide, but X and Y 
do not divide. They approach and touch each other, 
and are carried into the spindle, where they separate 
from each other when the other ordinary chromosomes 


Lygaeus © 





divide. Consequently there are formed two kinds of 
spermatozoa — one containing X and the other Y 
(Fig. 23, E). 

In the body cells and early germ-tract of the female 
of lygeus (Fig. 24, A), there are twelve ordinary 
chromosomes and two sex chromosomes, X and X. 
After reduction there are seven double chromosomes, 
the two X’s having united when the other chromosomes 


46 HEREDITY AND SEX 


united (B). Two divisions take place (C, D), when the 
two polar bodies are formed, leaving seven chromosomes 
in the egg (#). Each egg contains as a result only one 
X chromosome. 

Any egg of lygeeus fertilized by a sperm carrying an 
X chromosome produces a female that contains two 


S _ Ady 


@ee 
@° ee e*°e —— @0000eee 


@0ee@ ee sphere 
ote Gee te 
A J \ Ni 
E 


eee 
&o3— Soee°'see 


bs Y \\\\ i // fl 
\\ / / | | i 
z \\ Yn Oe ral Ali sa 


\ 
\ 
e0ee088e 
os bad 


YM LZ 
Cc Nl a Et 
or. 


linge, YaGy, EZ 


Oncopellus 3 


X’s or XX. Any egg fertilized by a sperm containing 
a Y chromosome produces a male that contains one 
X and one YsorexX Yo 

Another insect, Oncopeltus fasciatus, represents a 
third type in which the chromosome groups in the 
male and in the female are numerically alike and alike 
as to visible size relations. 


THE MECHANISM OF SEX-DETERMINATION = 47 


In the body cells of the male there are sixteen chro- 
mosomes (Fig. 25, A). After reduction there are nine 
chromosomes — seven in a ring and two in the middle 
(B). The seven are the fused pairs or double chro- 
mosomes; the two in the middle are the sex chromo- 
somes that have not fused. 


Oncopettus ¢ 





Fig. 26. 


The evidence for this interpretation is circumstan- 
tial but sufficient. 

At the first reduction division all nine chromosomes 
divide (C). Just before the second division the two 
central chromosomes come together and remain in 
contact (DD’). All the double chromosomes then 
divide, while the two sex chromosomes simply sepa- 
rate from each other, so that there are eight chromo- 
somes at each pole (DE). 3 


48 HEREDITY AND SEX 


In this case all of the spermatozoa (HE’) contain 
eight chromosomes. There is no visible difference 
between them. Nevertheless, there is reason for be- 
lieving that here also there are two kinds of sperm. 
The principal reason 1s that, there are all connecting 
stages between forms in which there is an unequal pair, 


Ascaris 3 
ve US 


7 
‘A i WW 
be Ai LYN® 


eS 
B \\ 





as in lygeeus, and forms with an equal pair, as in oncopel- 
tus. Another reason is that the two sex chromosomes 
behave during the synapsis stages as do the X Y chromo- 
somes in related species. Moreover, the experimental 
evidence, of which I shall speak later, leads us to con- 
clude that the determination of sex is not due only to 


THE MECHANISM OF SEX-DETERMINATION 49 


a difference in size of X and Y. The sex chromosomes 
must carry a host of factors other than those that de- 
termine sex. Consequently it is not surprising that in 
many species the sex chromosomes appear equal or 
nearly equal in size. It is a fortunate circumstance for 
us that in some species there is a difference in size or 


AsCcarts 9 






NS 
wd) 
Fig. 28. 


an unpaired sex chromosome; for, in consequence, we 
are able to trace the history of each kind of sperm in 
these cases; but it is not essential to the theory that 
X and Y, when present, should be visibly different. 

In the female of oncopeltus sixteen chromosomes 
occur as in the male (Fig. 26, 4). The reduced number 
is eight double chromosomes (B). At one of the two 
polar divisions eight chromosomes pass out, and eight 
remain in the egg (C). At the second division also 
eight pass out, and eight remain in the egg (D). 


00 HEREDITY AND SEX 


I shall pass now to a fourth condition that has only 
recently come to light. It is best shown in some of the 
nematode worms, for example, in the ascaris of the 
horse. Here the sex chromosomes are generally at- 
tached to other chromosomes. In this case, as shown 
by the diagram (Fig. 27, A), there is in the male a single 
X attached to one of the other chromosomes. At the 
first spermatocyte division it does not divide (C), 
but passes over bodily to one pole, so that two kinds 
of cells are produced. At the second spermatocyte 
division it divides, in the cell that contains it, so that 
each daughter cell gets one X (D). Two classes of 
sperm result, two with X (//), two without (H’). 

In the female there are two X’s, each attached to a 
chromosome (Fig. 28). After the polar bodies are 
given off, one X only is left in each egg (C, D, FH). Sex 
is determined here in the same way as in the insects, 
described above, for there are two classes of sperm and 
but one class of eggs. 

The discovery of the sex chromosome and its rela- 
tion to sex is due to several investigators. In 1891 
Henking first described this body, and its unequal distri- 
bution, but was uncertain even as to its relation to the 
chromosomes. Paulmier (1899), Montgomery (1901), 
Sinéty (1901), gave a correct description of its behavior 
in spermatogenesis. McClung (1902) confirmed these 
discoveries, and suggested that the accessory, or odd 
chromosome, as it was then called, had some relation 
to sex, because of its unequal distribution in the 
sperms. He inferred that the male should have one 
~more chromosome than the female, but he gave no evi- 
dence in support of this suggestion, which as we have 


THE MECHANISM OF SEX-DETERMINATION dl 


seen is the reverse of the actual conditions. Stevens 
(1905) made out the relations of the X Y pair of chro- 
mosomes to sex and Wilson in the same year (1905) 
the correct relation of the accessory chromosome to sex. 
The results described above for the insects are for the 
most part from Wilson’s studies on the chromosomes ; 
these for ascaris from the recent work of Sophia 
Frolowa, which confirms in the main the work of Boveri, 
Gulick, Boring, and Edwards. 

A case similar to ascaris has been described by Stevens 
for the mosquito, in which there is an XY and a Y in the 
male, each attached to another chromosome. In the 
guinea pig also, there seems to be in the male an X and 
a Y, attached to another pair of chromosomes.  Find- 
ing these cases so widely distributed, it seems not un- 
likely that in other cases, where an unpaired X or an 
X and a Y have not been detected, they are parts of 
other chromosomes. 

The whole history of the sex chromosomes of ancyro- 
canthus, a nematode worm, is strikingly shown in a 


recent paper by Carl Mulsow (Fig. 29 and 29a, A). 
This is a typical case in which the male has one less, 
chromosome than the female, as in protenor. The 


case is striking because the chromosomes can be seen 
and counted in the living spermatozoa. Some sperm 
have six, some have five chromosomes. The sperm- 
nucleus can be identified in the egg after fertilization 
because it lies nearer the pole opposite to the polar 
bodies. The entering sperm nuclei show in half of 
the fertilized eggs six chromosomes and in the other 
half five chromosomes. 

An interesting confirmation of these conclusions in 


o2 HEREDITY AND SEX 


regard to the relation between sex and the sex chromo- 
somes was found in another direction. It has long been 
known that the fertilized eggs of aphids or plant lice 
produce only females. The same thing happens in 
near relatives of the plant lice, the phylloxerans. 


ot 
° X - e . 
CN ey os 
h ? ?* 
Pe 
fons “ : “.@ 
eset . ee 
S of, 
¢ Ad 
e* 
ity, # 
“ " ee 
- ‘ 8 
ae © 
bead * 
ee * 
: °° a OF: 
“ee xs 
e 
eof ‘ Ps 
y ° 
a @ ° ’ 
mie 5 3 “eo 
Ped * Dy ‘ast ‘on 
se 
. - 2s “i &, 
s . et 
ee 4 % <7 { we 
i, Ay P| 
‘ 
rh 4 2 bd +e ber) 
e +? fee a 
1” 
Fic. 29.—1 and 2 are spermatogonia; 3, growth period; 4-7, prophases; 


8, equatorial plate of first division, 9-10; 11, spermatocytes of second order ; 
12-13, division of same; 14-16, the four cells or spermatids that come 
from the same original cell, two with 5, two with 6 chromosomes; 17, 
spermatids; 18, mature sperm; 19, living sperm. (After Mulsow.) 


In these insects a study of the chromosomes shows 
that the male has one less chromosome than the female. 
At the first maturation division in the male (Fig. 30), 
all the chromosomes divide except one, the X chromo- 
some, and this passes to one cell only. This cell is 
also larger than the sister cell. The small cell lacking 
the X degenerates, and does not produce spermato- 


THE MECHANISM OF SEX-DETERMINATION — 53 


zoa. The large cell divides again, all of the chromo- 
somes dividing. Two functional spermatozoa are 
produced, each carrying one sex chromosome. These 
spermatozoa correspond to the female-producing sper- 
matozoa of other insects. 

In the sexual female there is an even number of chro- 





Fig. 29a. — 20 and 21, odgonia (equatorial plate); 22, growth period; 
23, before fertilization; 24-25, entrance of sperm; 26-31, prophases of 
first division; 32-33, formation of first polar body; 34-36, extrusion of 
same and formation of second polar body; 37, two pronuclei; 38-41, union 
of pronuclei; 42-45, cleavage. (After Mulsow.) 


mosomes — one more than in the male. They unite 
in pairs. When the two polar bodies of the sexual 
ege are formed, all the chromosomes divide twice, so 
that each egg is left with one sex chromosome. 

It is now evident why only females are produced 
after fertilization. The female-producing sperm alone 
is functional. 


54 HEREDITY AND SEX 


Second Spermatocyle: 


O_O 
@) 


Wem. 


Fiag. 30.— Diagram of chromosomes in Phylloxera caryecaulis. Top 
line, somatic cell of female with 6 chromosomes and somatic cell of male 
with 5 chromosomes. Second line, stages in first spermatocyte division 
producing a rudimentary cell (below) with two chromosomes. Third line, 
second spermatocyte division into two equal cells. Fourth line, sexual 
egg (3 chromosomes) and two polar bodies; and two functional, female- 
producing sperm with three chromosomes each. 


© 


0 6) 


Fitst wWShermaloeyl, 


@ @ 
22) 


THE MECHANISM OF SEX-DETERMINATION 59 


THE EXPERIMENTAL EVIDENCE 


The experimental evidence, indicating that there is 
an internal mechanism for sex determination, is derived: 
from two sources — from experimental embryology, and 
from a study of the heredity of sex-linked characters. 

The evidence from embryology shows that the chro- 
mosomes are the bearers of materials essential for the 
production of characters. The evidence from hered- 
ity shows that certain characters follow the sex 
chromosomes. 

It has long been taught that the hereditary factors 
are carried by the nucleus. The evidence for this was 
found in fertilization. When the spermatozo6n enters 
the egg, it carries in, as a rule, only the head of the sper- 
matozoon, which consists almost entirely of the nucleus 
of the original cell from which it comes. Since the 
male transmits his characters equally with the female, 
it follows that the nucleus is the source of this 
inheritance. 

The argument has not been regarded as entirely 
conclusive, because the sperm may also bring in some 
of the protoplasm of the original cell—at least that part 
lying immediately around the nucleus. In addition a 
small body lying at the base of the sperm head seems 
also to be brought in by the male, and according to 
some observers it becomes the center about which the 
entire division system or karyokinetic spindle develops. 

The most convincing evidence that the chromosomes 
are the most important elements in heredity is found in 
some experimental work, especially that of Boveri, 
Baltzer, and Herbst. Under certain circumstances in 


56 HEREDITY AND SEX 


the sea-urchin two spermatozoa may enter a single 
egg. They both unite with the egg nucleus (Fig. 31). 
Each brings in 18 chromosomes. ‘The egg contributes 
18 chromosomes. There are in all 54, instead of 36 
chromosomes, as in normal fertilization. 





Fic. 31. — Dispermy and its effects in egg of sea urchin. (After 


Boveri.) 


Around these chromosomes a double system of 
threads develops with four poles. The chromosomes 
become unequally distributed on the four spindles that 
develop. Each chromosome then divides, and half of 
each goes to the nearest pole. To some of the poles 
many chromosomes may pass, to other poles fewer. 


THE MECHANISM OF SEX-DETERMINATION ‘57 


In order to simplify the case let us imagine that each 
sperm has only four chromosomes and the egg nucleus 
only four. Let us represent these by the letters as 
shown in Fig. 32. Any one of the four cells that is 





Fig. 32. — Diagram illustrating the irregular distribution of the chro- 
mosomes in dispermic eggs in an imaginary case with only four kinds of 
chromosomes, a, b, c, d. There are here three sets of each of these in 
each egg. The stippled cells are those that fail to receive one of each 
kind of chromosome. (After Boveri.) 


produced at the first division of these dispermic eggs 
may contain a full complement of the chromosomes, 
or only some of them. The possibilities for four 
chromosomes are shown in the diagram. <Any cell 
that does not contain at least these four chromosomes 
is shaded. One case is present in which all the four 


08 HEREDITY AND SEX 


cells contain a complete assortment. If normal devel- 
opment depends on an embryo containing in every cell 
at least one of each kind of chromosome, then in 
our simple case only one group of four cells has this 
possibility. 

Boveri found that such dispermic eggs produce 
normal embryos very rarely. He calculated what the 
chance would be when three times 18 chromosomes 
are involved. The chance for normal development 
is probably not once in 10,000 times. He isolated 
many dispermic eggs and found that only one in 1,500 
of the tetrad type developed normally. 

Boveri went still further in his analysis of the prob- 
lem. It had been shown for normal eggs that if at 
the two-celled stage the cells are separated, each forms 
a perfect embryo. This is also true for each of the 
first four cells of the normal egg. 

Boveri separated the four cells of dispermic eggs and 
found that the quadrants not infrequently developed 
normally. This is what we should anticipate if those 
cells can develop that contain one of each kind of 
chromosome. 

The evidence furnishes strong support of the view 
that the chromosomes are different from each other, 
and that one of each kind is necessary if development 
is to take place normally. 

The evidence that Baltzer has brought forward is 
also derived from a study of sea-urchin eggs. It is 
possible to fertilize the eggs of one species with sperm 
of another species. The hybridizing is greatly helped 
by the addition of a little alkali to the sea water. 

Baltzer made combinations between four species of 


THE MECHANISM OF SEX-DETERMINATION 59 


sea-urchins. We may take one cross as typical. When 
eges of strongylocentrotus are fertilized with sperm of 
spherechinus, it is found at the first division of the egg 
that, while some of the chromosomes divide and pass 
normally to the two poles, other chromosomes remain 
in place, or become scattered irregularly between the 
two poles, as shown in Fig. 338. When the division 





Vain whe let LW g purty) a om 


oN bay 
era ie 


Mad ae wR dat 
4 is 


it 
J Ye 
, if 
V4 ‘wf 
yaw xX\NN NU7 
aa WHA A ca ier 
wey oe ‘2 Fy ee ~ 
$i ‘1 ' 4 a, TNSS 
& ga ‘ 
Fic. 33. — 1 and la, chromosomes in the normal first cleavage spindle of 


Spherechinus; 2, equatorial plates of two-cell stage of same; 3—3a, hybrid, 
Spherechinus by Strongylocentrotus, spindle at two-cell stage; 4-4a, same 
equatorial plates; 5—5a, hybrid, Strong. by Spheer., cleavage spindle in telo- 
phase; 6, next stage of last; 7, same, two-cell stage; 8, same, later; 9, same, 
four-cell stage; 10, same, equatorial plate in two-cell stage (12 chromosomes) ; 
11, same, from later stage, 24 chromosomes. (After Baltzer.) 


is completed, some of these chromosomes are found 
outside of the two main nuclei. They often appear 
as irregular granules, and show signs of degeneration. 
They are still present as definite masses after the next 
division, but seem to take no further part in the de- 
velopment. 

Baltzer has attempted to count the number of chro- . 
mosomes in the nuclei of these hybrid embryos. The 


60 HEREDITY AND SEX 


number is found to be about twenty-one. The maternal 
egg nucleus contains eighteen chromosomes. It appears 
that only three of the paternal chromosomes have 
succeeded in getting into the regular cycle — fifteen of 
them have degenerated. 

Baltzer thinks that the egg acts injuriously in this 
case on the chromosomes of foreign origin, especially 
on the fifteen that degenerate, so that they are elim- 
inated from the normal process. 

The embryos that develop from these eggs are often 
abnormal. <A few develop as far as the pluteus stage, 
when a skeleton appears that is very characteristic for 
each species of sea-urchin. The plutei of these hybrids 
are entirely maternal. This means that they are 
exactly like the plutei of the species to which the 
mother belongs. 

The conclusion is obvious. The sperm of spherechi- 
nus has started the process of development, but has 
produced no other effect, or has at most only slightly 
affected the character of the offspring. It is reason- 
able to suppose that this is because of the elimination 
of the paternal chromosomes, although the evidence 
is not absolutely convincing. 

Let us now examine the reciprocal cross. When the 
eges of sphearechinus are fertilized by the sperm of 
strongylocentrotus, the division of the egg and of the 
chromosomes is entirely normal. All the chromosomes 
divide and pass to the poles of the spindle. The total 
number (36) must, therefore, exist in each cell, although 
in this case they were not actually counted. 

The pluteus that develops has peculiarities of both 
maternal and paternal types. It is hybrid in structure. 


THE MECHANISM OF SEX-DETERMINATION 61 


Both parents have contributed to its formation. It is 
not going far from the evidence to infer that the hybrid 
character is due to both sets of chromosomes being 
present in all of the cells. 





Fic. 34. — 1. The chromosomes of the egg lie in the equator of the 
spindle, the chromosomes of the sperm lie at one side. 2. A later stage, 
showing all the paternal chromosomes passing to one pole. 3 (to the right). 
A later stage, a condition like the last. There is also a supernumerary sperm 
in the egg (to left, in another section.) 4. Same condition as last. 5. Plu- 
teus larva that is purely maternal on one side and hybrid on the other. 
(After Herbst.) 


The evidence that Herbst has brought forward is of 
a somewhat different kind. It supplements Baltzer’s 
evidence and makes more probable the view that the 
chromosomes are essential for the development of the 
characters of the individual. 


62 HEREDITY AND SEX 


Herbst put the eggs of spherechinus into sea water 
to which a little valerzanic acid had been added. This 
is one of the many methods that Loeb has discovered 
by which the egg may be induced to develop parthe- 
nogenetically, 7.e. without the intervention of the sper- 
matozoon. After five minutes the eggs were removed to 
pure sea water and the sperm of another species, stron- 
gylocentrotus, was added. ‘The sperm penetrated some 
of the eggs. The eggs had already begun to undergo 
the changes that lead to division of the cell — the sperm 
entered ten minutes late. The egg proceeded. to 
divide, the sperm failed to keep pace, and fell behind. 
Consequently, as shown in Fig. 34, the paternal 
chromosomes fail to reach the poles when the nuclei 
are re-formed there. The paternal chromosomes form 
a nucleus of their own which comes to lie in one of the 
two cells. In consequence one cell has a nucleus that 
contains only the maternal chromosomes; the other 
cell contains two nuclei, one maternal and the other 
paternal. In later development the paternal nucleus 
becomes incorporated with the maternal nucleus of its 
cell. There is produced an embryo which is maternal 
on one side and hybrid on the other. Herbst found in 
fact that half-and-half plutei were not rare under the 
conditions of his experiment. 

This evidence is almost convincing, I think, in 
favor of the view that the chromosomes are the es- 
sential bearers of the hereditary qualities. For in 
this case, whether the protoplasm of the embryo 
comes from the egg or the sperm also, the fact re- 
mains that the half with double nuclei is hybrid. 
Even if the spermatozo6dn brought in some _ proto- 


THE MECHANISM OF SEX-DETERMINATION' 63 


plasm, there is no reason to suppose that it would 
be distributed in the same way as are the paternal 
chromosomes. | 


EVIDENCE FROM SEX-LINKED INHERITANCE 


The experimental evidence based on sex-linked in- 
heritance may be illustrated by the following examples. 

The eyes of the wild fruit-fly, Drosophila ampe- 
lophila, are red. In my cultures a male appeared that 
had white eyes. He was mated to a red-eyed female. 
The offspring were all red-eyed — both males and 
females (Fig. 35). These were inbred and produced 
in the next generation red-eyed females, red-eyed males, 
and white-eyed males (Fig. 35). There were no white- 
eyed females. The white-eyed grandfather had trans- 
mitted white eyes to half of his grandsons but to none 
of his granddaughters. 

Equally important are the numerical proportions 
in which the colors appear in the grandchildren. There 
are as many females as the two classes of males taken 
together; half of the males have red eyes and _ half 
have white eyes. The proportions are therefore 50% 
red females, 25 % red males, 25 % white males. 

Only white-eyed males had appeared at this time. 
It may seem that the eye color is confined to the male 
sex. Hence the origin of the term sea-lamited inheri- 
tance for cases like this. But white-eyed females may 
be produced easily. ae some of the red-eyed grand- 
daughters are bred to these white-eyed males, both 
white-eyed females and males, and red-eyed females 
and males, appear (Fig. a7). )a wnite eye is there- 
fore not sex-limited but sex-linked. 


~ 


64 HEREDITY AND SEX 


With these white-eyed females it is possible to make 
the reciprocal cross (Fig. 36). A white-eyed female 
was mated to a red-eyed male. All of the daughters 
had red eyes and all the sons had white eyes. These 
were then inbred and gave red-eyed males and females, 





Fia. 35. — Sex-linked inheritance of white and red eyes in Drosophila. 
Parents, white-eyed ¢ and red-eyed Q; Fi, red-eyed @ and Q; F» red- 
eyed Q, red-eyed 8 and white-eyed @. To right of flies the history of the 
sex chromosomes XX is shown. The black X carries the factor for red 
eyes, the open X the factor for white eyes; the circle stands for no X. 
and white-eyed males and females in equal numbers 
(Fig. 36). 

The heredity of this eye color takes place with the 
utmost regularity, and the-results show that in some 
way the mechanism that is involved is closely bound 


up with the mechanism that produces sex. 


THE MECHANISM OF SEX-DETERMINATION 65 


Other combinations give results that are predictable 
from those just cited. For instance, if the /, red-eyed 
female from evther of the preceding crosses is mated to 
a white-eyed male, she produces red-eyed males and 
females, and white-eyed males and females, as shown in 


XO) XK 


KO XX 


» COOK XC 


Fic. 36. — Reciprocal cross of Fig. 35. Parents, white-eyed 2 and 
red-eyed ¢, (criss-cross inheritance). Fi, red-eyed &, white-eyed ¢. 
Fh, red-eyed Q and @; white-eyed @ and ¢. To right, sex chromo- 
somes (as in Fig. 35). 





Fig. 37 (upper two lines). If the /; red-eyed male 
from the first cross (Fig. 35) is bred to a white-eyed 
female, he will produce red-eyed daughters and white- 
eyed sons. Fig. 37 (lower two lines). 

The same relations may next be illustrated by an- 
other sex- -linked’ character. 


66 HEREDITY AND SEX 


A male with short or miniature wings appeared in 
my cultures (Fig. 38). Mated to long-winged females 
“only long-winged offspring were produced. When 
these were mated to each other, there were produced 


X 

















Fie. 37. — Upper series, back cross of Fi 2 to white ¢. Lower series 
back cross of F; red-eyed g@ to white @. 
long-winged females (50%), long-winged males (25%) 
and miniature-winged males (25%). 
It is possible to produce, in the way described for 
eye color, miniature-winged females. 
When such miniature-winged females are mated to 
long-winged males, all the daughters have long wings, 
and all the sons have miniature wings (Fig. 39). If 


THE MECHANISM OF SEX-DETERMINATION 67 


these are now mated, they produce, in equal numbers, 
long-winged males and females and miniature-winged 
males and females. 

The same relations may again be illustrated by body 
color. 


XO XX 


X0) Xx 





Fig. 38. — Sex-linked inheritance of short (“‘miniature’’) and long’ wings ' 
in Drosophila. Parents, short-winged ¢, long-winged 2. F, long-winged 
$6 and 2. F. long-winged Q and ¢ and short-winged @. Sex chromo- 
somes to right. Open X carries short wings. 


A male appeared with yellow wings and body. Mated 
to wild gray females he produced gray males and 
females. These mated to each other gave gray females 
(50%), gray males (25%), and yellow males (25%). 

As before, yellow females were made up. Mated to 
eray males they gave gray females and yellow males. 


68 HEREDITY AND SEX 


These inbred gave gray males and females and yellow 
males and females, in equal numbers. 

These cases serve to illustrate the regularity of this 
type of inheritance and its relation to sex. In the fruit 
fly we have found as many as twenty-five sex-linked 


XO 


XO XK 





Fig. 39. — Reciprocal cross of Fig. 38. Parents, long-winged ¢@ and 
short-winged 9. F, long-winged @, short-winged @. F: long-winged 
9 and @, short-winged 9 and ¢. Sex chromosomes as in last. 


factors. There are other kinds of inheritance found in 
these flies, and at another time I shall speak of some 
of these ; but the group of sex-linked factors is of special 
importance because through them we get an insight 
into the heredity of sex. 

In the next chapter, when we take up in detail Men- 
delian heredity, I shall try to go further into the ex- 


THE MECHANISM OF SEX-DETERMINATION 69 


planation of these facts. For the present it will suffice 
to point out that the cases just described, and all like 
them, may be accounted for by means of a very simple 
hypothesis. We have traced the history of the sex 
chromosomes. If the factors that produce white eyes, 
short (miniature) wings, and yellow body color are 
carried by the sex chromosomes, we can account for 
these results that seem at first sight so extraordinary. 
The history of the sex chromosomes is accurately known. 
Their distribution in the two sexes is not a matter of 
conjecture but a fact. Our hypothesis rests therefore 
on a stable foundation. 

At the risk of confusion I feel bound to present here 
another type of sex-linked inheritance. In principle 
it is like the last, but the actual mechanism, as we shall 
see, 1s somewhat different. Again I shall make use 
of an illustration. If.a black Langshan hen is mated 
to a barred Plymouth Rock cock, all the offspring will 
be barred (Fig. 40). If these are inbred, there are pro- 
duced barred females and males, and black females. 
The numerical proportion is 50 per cent barred males, 25 
per cent barred females, and 25 per cent black females. 

The black hen has transmitted her character to half 
of her granddaughters and to none of her grandsons. 
The resemblance to the case of the red-eyed, white- 
eyed flies is obvious, but here black appears as a sex- 
linked character in the females. | 

The converse cross is also suggestive. When a 
barred hen is mated to a black cock, all the daughters 
are black and all the sons are barred (Fig. 41). When 
these are inbred, there are produced black males and 
females and barred males and females in equal num- 


70) HEREDITY AND SEX 


bers. Again, the resemblance of the reciprocal cross 
to one of the combinations for eye color is apparent. 
In fact, this case can be explained on the same prin- 
ciple as that used for the flies, except that in birds it is 





Parents 





Fia. 40. — Sex-linked inheritance in fowls. Upper Jine black Laugshan 
hen and barred Plymouth Rock cock. Second live, Fi, barred cock and 
hen. Third line, Ms, three barred (cock, hen, cock) and one black (hen). 
(Cuts from *‘ Reliable Poultry Journal.’’) 


THE MECHANISM OF SEX-DETERMINATION 71 


the female that produces two kinds of eggs; she is 
heterozygous for a sex factor while the male produces 
only one kind of spermatozo6n. 





Fic. 41.— Reciprocal cross of Fig. 40. Upper line, black cock and 
barred hen. Second line, Ff), barred cock and black hen. Third line, FP», 
barred hen and cock, black cock and hen. (Cuts from ‘‘ Reliable Poultry 
Journal.’’) 


72 HEREDITY AND SEX 


We lack here the certain evidence from cytology that 
we have in the case of the insects. Indeed, there is 
some cytological evidence to show that the male bird 
is heterozygous for the sex chromosome. But the 
evidence does not seem to me well established ; while 
the experimental evidence is definite and has been 
independently obtained by Bateson, Pearl, Sturtevant, 
Davenport, Goodale and myself. However this may be, 
the results show very clearly that here also sex is con- 
nected with an internal mechanism that is concerned 
with other characters also. This is the mechanism of 
Mendelian heredity. Whether the chromosomes suffice 
or do not suffice to explain Mendelian heredity, the 
fact remains that sex follows the same route taken by 
characters that are recognized as Mendelian. 

To sum up: The facts that we have considered 
furnish, I believe, demonstrative evidence in favor 
of the view that sex is regulated by an internal mech- 
anism. The mechanism appears, moreover, to be the 
same mechanism that regulates the distribution of cer- 
tain characters that follow Mendel’s law of inheritance. 


SS —— 


GTAP PMR Ler 


THE MENDELIAN PRINCIPLES OF HEREDITY AND 
THEIR BEARING ON SEX 


THE modern study of heredity dates from the year 
1865, when Gregor Mendel made his famous discoveries 
in the garden of the monastery of Briinn. For 35 
years his paper, embodying the splendid results of his 
work, remained unnoticed. It suffered the fate that 
other fundamental discoveries have sometimes met. 
In the present case there was no opposition to the 
principles involved in Mendel’s discovery, for Darwin’s 
great work on “ Animals and Plants”’ (1868), that dealt 
largely with problems of heredity, was widely read and 
appreciated. True, Mendel’s paper was printed in 
the journal of a little known society — the Natural 
History Society of Brinn, — but we have documentary 
evidence that his results were known to one at least of 
the leading botanists of the time. 

It was during these years that the great battle for 
evolution was being fought. Darwin’s famous book on 
“The Origin of Species”? (1859) overshadowed all else. 
Two systems were in deadly conflict — the long-ac- 
cepted doctrine of special creation had been challenged. 
To substitute for that doctrine the theory of evolution 
seemed to many men of science, and to the world at 
large, like a revolution in human thought. It was in 
fact a great revolution. The problems that bore on the 

73 





74 HEREDITY AND SEX 


question of how the higher animals and plants, and 
man himself, arose from the lower forms seemed the 
chief goal of biological work and thought. The out- 
come was to establish the theory of evolution. The 
circumstantial evidence that was gathered seemed so | 
fully in accord with the theory of evolution that the 
theory became widely accepted. The acute stage was 
passed, and biologists found themselves in a position 
to examine with less haste and heat many other phe- 
nomena of the living world equally as important as 
evolution. 

It gradually became clear, when the clouds of con- 
troversy had passed, that what I have ventured to call 
the ‘“‘ circumstantial evidence”? on which the theory of 
evolution so largely rested, would not suffice as a direct 
proof of evolution. Investigation began to turn once 
more to that field of observation where Darwin had 
found his inspiration. The causes of variations and 
the modes of inheritance of these variations, the very 
foundations of the theory of evolution, were again 
studied in the same spirit in which Darwin himself had 
studied them. The return to Darwin’s method rather 
than to Darwin’s opinions marks the beginning of the 
new era. 

In 1900 three botanists were studying the problem 
of heredity. Each obtained evidence of the sort 
Mendel had found. Happily, Mendel’s paper was 
remembered. The significance of his discovery now 
became apparent. De Vries, Correns, and Tschermak 
brought forward their evidence in the same year (1900). 
Which of the three first found Mendel cannot be stated, 
and is of less importance than the fact that they ap- 





THE MENDELIAN PRINCIPLES OF HEREDITY ,75 


preciated the significance of his work, and realized 
that he had found the key to the discoveries that they 
too had made. From this time on the recognition of 
Mendel’s discovery as of fundamental importance was 
assured. Bateson’s translation of his paper made 
Mendel’s work accessible to English biologists, and 
Bateson’s own studies showed that Mendel’s principles 
apply to animals as well as to plants. 


THE HEREDITY OF ONE PAIR OF CHARACTERS 


Mendel’s discovery is sometimes spoken of as Men- 
del’s Principles of Heredity and sometimes as Mendel’s 
Law. The former phrase gives a better idea perhaps 
of what Mendel really accomplished, for it is not a little 
difficult to put his conclusions in the form of a law. 
Stated concisely his main discovery is this:—din the 
germ-cells of hybrids there is a free separation of the 
elements derived from the two parents without regard to 
which parent supplied them. 

An example will make this more obvious. Mendel 
crossed an edible pea belonging to a race with yellow 
seeds to a pea belonging to a race with green seeds 
(Fig. 42). The offspring or first filial generation (F) 
had seeds all of which were yellow. When the plants 
that bore these seeds were self-fertilized, there were 
obtained in the next generation, /,, both yellow and 
green peas in the proportion of 3 yellows to 1 green 
(Fig. 42). This is the well-known Mendelian ratio 
Bip l: 

The clue to the meaning of this ratio was found when 
the plants of the second generation (/2) were selfbred. 
The green peas bred true; but the yellows were of two 


~* 


76 HEREDITY AND SEX 


kinds — some produced yellow and green seeds again 
in the ratio of 3:°1; others produced only yellow 
peas. Now, if the yellows that bred true were counted, 
it was found that the number was but one-third of 
all the yellows. 






PARENTS 





Fig. 42. — Illustrating Mendel’s cross cf yellow (lighter color) and green 
(dark color) peas. 


THE MENDELIAN PRINCIPLES OF HEREDITY 77 


In other words, it was shown that the ratio of 3 yel- 
lows to 1 green was made up of 1 pure yellow, 2 hy- 
brid yellows, 1 pure green. This gave a clue to the 
principles that. lay behind the observed results. 

Mendel’s chief claim to fame is found in the discovery 
of a simple principle by means of which the entire 
series of events could be explained. He pointed out 
that if the original parent with yellow (P;) carried 
something in the germ that made the seed yellow, and 
the original parent with green seeds (P;) carried some- 
thing that made the seed green, the hybrid should con- 
tain both things. If both being present one domi- 
nates the other or gives color to the pea, all the peas in 
the hybrid generation will be of one color — yellow in 
this case. Mendel assumed that in the germ-cells of 
these hybrids the two factors that make yellow and 
green separate, so that half of the germ-cells contain 
yellow-producing material, and half contain green- 
producing material. This is Mendel’s principle of 
separation or segregation. It is supposed to occur 
both in the male germ-cells of the hybrid flower, i.e. 
in the anthers, and also in the ovules. If self-fertili- 
zation occurs in such a plant, the following combina- 
tions are possible: A yellow-bearing pollen grain may 
fertilize a ‘‘yellow”’ ovule or it may fertilize a ‘‘green’”’ 
ovule. The chances are equal. If the former occurs, 
a pure yellow-seeded plant will result; if the latter a 
hybrid yellow-seeded plant. The possible combina- 
tions for the green-producing pollen are as follows: A 
“‘oreen’’ pollen grain may fertilize a ‘“‘yellow’”’ ovule, 
and produce a hybrid, yellow-seeded plant, or it may 
fertilize a ‘‘green”’ ovule, and produce a green-seeded 


78 HEREDITY AND SEX 


plant. If these meetings are random, the general or 
average outcome will be: 1 pure yellow, 2 hybrid 
yellows, and 1 pure green. 

It is now apparent why the pure yellows will always 
breed true, why the yellow-greens will split again into 
yellows and greens (or 1:2:1), and why the pure 
greens breed true. By this extremely simple assump- 
tion the entire cutcome finds a rational explanation. 


& PARENTS & 


e © 
se es 
@ @e 8» ‘O- 


Fia. 43. — “ Checker”’ diagram to show segregation and recombination of 
factors. In upper line, a black bearing gamete is supposed to unite with a 
white bearing gamete to give the zygotes shown in F, each of which is 
heterozygous for black-white here represented as allelomorphs, etc. 





The same scheme may be represented by means of 
the above ‘‘checker”’ diagram (Fig. 48). Black crossed 
to white gives hybrid black, #,, whose germ-cells are 
black or white after segregation. The possible com- 
bination of these on random meeting at the time of 
fertilization is shown by the arrows in F and the results 
are shown in the line marked F,. There will be one 
pure black, to two black-and-whites, to one pure white. 


THE MENDELIAN PRINCIPLES OF HEREDITY 79 


The first and the last will breed true, if self-fertilized, 
because they are pure races, while the black-and-whites 
will give once again, if inbred, the proportions 1:2: 1. 

A better illustration of Mendel’s principles is shown 
in the case of the white and red Mirabilis jalapa de- 
scribed by Correns. This case is illustrated in Fig. 44, 


te’ 










PARENTS 





Fic. 44. — Cross between white and red races of Mirabilis Jalapa, giving 
a pink hybrid in F; which when inbred gives, in Fs, 1 white, 2 pink, 1 red. 
in which the red flower is represented in black and the 
pink is in gray. The hybrid, F,, out of white by red, 
has pink flowers, 7.e. it is intermediate in color. When 
these pink flowers are self-fertilized they produce 
1 white, 2 pink, and 1 red-flowered plant again. The 
history of the germ-cells is shown in Fig. 45. The germ- 


SO HEREDITY AND SEX 


cell of the /, pink flower segregates into ‘“‘white’’ and 
“red,”’ which by chance combination give the white 
pink, and red flowers of F,. The white and red flowers 
are pure; the pink heterozygous, 7.e. hybrid or mixed. 
In this case neither red nor white dominates, so that the 
hybrid ean be distinguished from both its parents. 


G) PARENTS & 
at i 
2% a 








Fe C) 
| TS | 
“ O 6 © @ @ 


Fig. 45. — Illustrating history of gametes in cross shown in Fig. 44. A 
white and a red bearing gamete unite to form the pink zygote in F;, whose 
gametes, by segregation, are red and white, which by random combinations 
give the F, zygotes, etc. 








Mendel tested his hypotheses in numerous ways, that 
I need not now discuss, and found in every case that 
the results coincided with expectation. 
THE HEREDITY OF A SEX-LINKED CHARACTER 


We are now in a position to see how Mendel’s funda- 


THE MENDELIAN PRINCIPLES OF HEREDITY 81 


mental principle of segregation applies to a certain class 
of characters that in the last chapter I called ‘‘sex- 
linked’’ characters. 

Diagram 35 (page 64) will recall the mode of trans- 
mission of one of these characters, viz. white eyes. _ 

Let us suppose that the determiner for white eyes 
is carried by the sex chromosome. The white-eyed 
male has one sex chromosome of this kind. This sex 
chromosome passes into the female-producing spermato- 

zoon. 

- Such a spermatozoén fertilizing an egg of the red- 
- eyed fly gives a female with two sex chromosomes — 
one capable of producing red, one capable of producing 
white. The presence of one red-producing chromosome 
suffices to produce a red-eyed individual. 

When the F, female produces her eggs, the two sex 
chromosomes separate at one of the two maturation 
divisions. Half of the eggs on an average will contain 
the ‘‘white’’ sex chromosome, half the ‘‘red.’? There 
are, then, two classes of eggs. 

When the F; male produces his sperm, there are 
also two classes of sperm—one with the “red” 
sex chromosome (the female-producing sperm), and 
one without a sex chromosome (the male-producing 
sperm). 

Chance meeting between eggs and sperm will give 
the classes of individuals that appear in the second filial 
generation (F,). It will be observed that the Mendelian 
ratio of 3 red to 1 white appears here also. Segregation 
gives this result. 

The explanation that has just been given rests on 
the assumption that the mechanism that brings about 


82 HEREDITY AND SEX 


the distribution of the red- and the white-producing 
factors is the same mechanism that is involved in sex 
determination. On this assumption we can readily 
understand that any character that is dependent on the 
sex chromosomes for its realization will show sex- “linked 
inheritance. 

The reciprocal cross (Fig. 36) is equally instructive. 
If a white-eyed female is mated to a red-eyed male, 
all the daughters are Ted- eyed like the father, and all 
the sons are white-eyed like the mother. When these, » 
F,, flies are bred to each other there are produced red- 
eyed females (25%), white-eyed females (25%), red- 
eyed males (25%), and white-eyed males (25%). The 
explanation (Fig. 36; page 65) is in principle the 
same as for the other cross. If, for instance, we 
assume that the two X chromosomes in the white-eyed 
female carry the factors for white, all the eggs will 
carry one white-producing X. The red-eyed male will 
contain one X chromosome which is red-producing 
and passes into the female-producing sperm. ‘The 
other sperm will not contain any sex chromosome, and 
hence lacks the factors for these eye colors. When the 
female-producing sperm, that carries the factor for 
red, fertilizes a ‘‘white”’ egg, the egg will give rise to a 
female with red eyes, because of the presence of one 
red-producing chromosome. When the male-produc- 
ing sperm fertilizes any egg, a white-eyed son will be 
produced, because the single sex chromosome which 
he gets from his mother is white-producing. 

x The production of four classes of individuals in the 
second generation works out on the same scheme, as 
shown in the diagram. The inheritance of white and 


THE MENDELIAN PRINCIPLES OF HEREDITY 83 


red eyes in these cases is typical of all sex-linked in- 
heritance. In man, for instance, color blindness, so 
common in males and rare in females, follows the 
same rules. It appears that hemophilia in man and 
night-blindness are also examples of sex-linked in- 
heritance. These cases, as already stated, were formerly 
included under the term ‘‘seax-limited inheritance,” that 
implies that a character is limited to one sex, but we 
now know that characters such as these may be trans- 
ferred to the females, hence the term is misleading. 
Their chief peculiarity is that in transmission they ap- 
pear as though linked to the factor for sex contained in 
the sex chromosome, hence I prefer to speak of them as 
sex-linked characters. 

If our explanation is well founded, each sex-linked 
character is represented by some substance — some 
material particle that we call a factor in the sex 
chromosome. There may be hundreds of such materials 
present that are essential for the development of sex- 
linked characters in the organism. 

The sex chromosomes must contain, therefore, a 
large amount of material that has nothing whatever 
to do with sex determination; for the characters in 
question are not limited to any particular sex, although 
in certain combinations they may appear in one sex 
and not in the other. 

What then, have the sex chromosomes to do with sex ? 
The answer is that sex, like any other character, is due 
to some factor or determiner contained in these chro- 
mosomes. It is a differential factor of such a kind 
that when present in duplex, as when both sex chromo- 
somes are present, it turns the scale so that a female 


84 HEREDITY AND SEX 


is produced — when present in simplex, the result is 
to produce a male. , 
In other words, it is not the sex chromosomes as a 

whole that determine sex, but only a part of these chro- 
mosomes. Hence we can understand how sex is deter- 

- mined when an unequal pair of chromosomes is pres- 
ent, as in lygeus. The smaller chromosome lacks 
the sex differential, and probably a certain number of 
other materials, so that sex-linked inheritance is pos- 
sible here also. Moreover, in a type like oncopeltus, 
where the two sex chromosomes are alike in size, we 
infer that they too differ by the sex differential. If 
all the other factors are present, as their size suggests, 
sex-linked inheritance of the same kind would not be 
expected, but the size difference observable by the 
microscope is obviously too gross to make any such 
inference certain. We have come to see that it was a 
fortunate coincidence only that made possible the dis- 
covery of sex determination through the sex chromo- 
somes, because the absence of the sex factor alone in 
the Y chromosome might have left that chromosome 
in the male so nearly the same size as the X in the 
female that their relation to sex might never have been 
suspected. When, however, one of the sex chromosomes 
began to lose other materials, it became possible to 
identify it and discover that sex is dependent upon 
its distribution. 


\ THE HEREDITY OF TWO PAIRS OF CHARACTERS 


Mendel observed that his principles of heredity apply 
not only to pairs of characters taken singly, but to 
cases where two or more pairs of characters are involved. 


THE MENDELIAN PRINCIPLES OF HEREDITY 85 


An illustration will make this clear. There are races 
of edible peas in which the surface is round ; other races 
in which the surface is wrinkled. Mendel crossed a 
pea that produces yellow round seeds with one that pro- 
duces wrinkled green seeds. 

The result of this cross was a plant that produced 
yellow round peas (Fig. 46). Both yellow and round 
are therefore dominant characters. When these F; 
plants were self-fertilized, there were produced plants 
some of which bore yellow round peas, some yellow 
wrinkled peas, some green round peas and some green 
wrinkled peas. These were in the proportion of 
Uipeten: 1, 

The explanation of the result is as follows: One of 
the original plants produced germ-cells all of which 
bore determiners for yellow and for round peas, YR; the 
other parent produced gametes all of which bore deter- 
miners for green and for wrinkled, GW (Fig. 47). 
Their combination may be represented : 


YR by GW = YRGW 


The germ-cells of the hybrid plant YRGW produced 
germ-cells (eggs and pollen) that have either Y or G, 
and R or W. Expressed graphically the pairs, the 
so-called allelomorphs, are: 


Me R 


G W 


and the only possible combinations are YR, YW, GR, 
GW. When pollen grains of these four kinds fall on 
the stigma of the same kind of hybrid. plant whose 
ovules are also of the four kinds the following chance 
combinations are possible : 


86 HEREDITY AND SEX 





























Pe ey omens YR 
YR YW oy. GR GW 
YW YW YW YW 
YR YW GR GW 
mabea <i GR Tae Gham 
YR YW Cina GW 
eee a GW GW GW 
YR YW Ghee GW 












PARENTS 8) 





1 


Fic. 46.— Illustrating Mendel’s cross of yellow-round with green- -wrinkled 
peas. The figures show the peas of F; and F» in the latter in the charac- 
teristic Tati 01/9's3.2°S sol. 


THE MENDELIAN PRINCIPLES OF HEREDITY 87 


PARENTS 








oats (YR YW GR GW) sss: 
¢2-\YR YW GR «GW sper 





Fig. 47. — Illustrating the history of the gametes ot the cross represented 
in Fig. 46. The composition of the parents YR and GW and of the F; hybrid 
YRGW is given above. The four classes of ovules and of pollen are given in 
the middle of the figure. These by random combinations give the kinds of 
zygotes represented in the squares below. (One YR should be GR.) 


88 HEREDITY AND SEX 


In each combination in the table the character of 
the plant is determined by the dominant factors, in 
this case yellow and round, hence: 


OY Sey Wee 316 ae 


This result works out on the assumption that there 
is independent assortment of the original determiners 
that entered into the combination. The determiner 
for yellow and the determiner for round peas are 
assumed to act independently and to segregate from 
green and wrinkled that entered from the other parent. 
The 9:3:3:1 ratio rests on this assumption and is the 


actual ratio realized whenever two pairs of characters _ 
freely Mendelize. J 


THE HEREDITY OF TWO SEX-LINKED CHARACTERS 


The inheritance of two sex-linked characters may be 
illustrated by an imaginary case in which the linkage 
of the factors to each other is ignored. Then the same 
case may be given in which the actual results obtained, 
involving linkage, are discussed. 

The factors in the fruit fly for gray color, G, and for 
red eye, R, are both sex-linked, 7.e. contained in the 
X chromosome. ‘Their allelomorphs, viz., yellow color, 
Y, and white eye, W, are also sex-linked. When a 
gray red-eyed female is mated to a yellow white-eyed 
male, the daughters and sons are gray-red, Gh. Their 
origin 1s indicated in the following scheme : 


Gray-red ? GR X—GR X 
Yellow-white 3 Ywx- ae 
F, GR X\'Y W X Gray-red 9 
Gag 2 eX Gray-red ¢ 


THE MENDELIAN PRINCIPLES OF HEREDITY | 89 


In the gray-red F’; female there will be the possibility 
of interchange of the G and Y, and of the W and R 
factors, so that gametes of four kinds will be formed, 
namely, GRX — GWX — YRX — YWX. For the 
moment we may assume free interchange of factors ; 
and therefore these four classes of eggs will exist in 
equal numbers. 

In the gray-red F; male there is but one X chromo- 
some that contains the factors G and R. There will 
be then only one kind of female-producing sperm, 
viz.. GRX; and one kind of male-producing sperm, 
the latter containing no X, and therefore none of the 
factors in question. The chance meeting of these two 
classes of sperm and the four classes of eggs gives the 
following results : 


Fi eggs GRX— GWX — YRX — YWX 
Fi, sperm GRX 





Females. Males. 
GRXGRX _ gray-red.— GRX_ gray-red. _ 
GRXGWX. gray-red.— GWX _ gray-white. 
GRXYRX _ gray-red.— YRX_ yellow-red. 
GRXYWX gray-red.— YWX yellow-white. 


All the females are gray with red eyes, since these 
are the dominant characters. There are four classes 
of males. These males give a measure of the kinds of 
eggs produced by the females, since the male-producing 
sperms, having no sex chromosomes, do not affect 
the sex-linked characters derived through the sex 
chromosome of the /; female. The expected proportion 
is therefore: 


90 HEREDITY AND SEX 


GR GR GW és YRS$ YWs 
4 1 1 1 1 


These results are illustrated by means of Fig. 48, 
in which the yellow color of the fly is indicated by 
stippling the body and wings, and the red eyes by 
black. The X chromosome is also marked and colored 








XX AX)AO XO 


f2 


Fria. 48. — Inheritance of yellow-white (3) and gray-red (2) of Dro- 
sophila. In F, both sexes are gray-red. In F: are produced 4 GR Q — 
LGRid —TGW. 3s YR SY Wes. 


THE MENDELIAN PRINCIPLES OF HEREDITY 91 


in the same way as the flies; thus the two X’s in the 
red-eyed gray female are half black (for red) and half 
eray; the single X in the white-eyed yellow male is 
half white and half stippled. 

In the F, generation the X chromosomes are first 
represented as they came in (second line), 7.e. with 
their original composition. The next line gives the 
three large classes that result, viz.. 2 GR? —1GRé¢ 
—1/YWeé. But if free interchange takes place in the 
female, some of the eggs will have chromosomes like 
those in the fourth line, viz. YR and GW. Such 
os will give the classes represented in the lowest line, 

,2GRe—1GWé—1 YR. Thus, as already 
Sane there results one kind of female and Our 
kinds of males. 

I said that the proportion 4:1:1:1:1 is the ideal 
result in the cross between the yellow-white and the 
gray-red flies, This ideal scheme is not realized because 
of a complication that comes in. ‘The complication 
is due to linkage or a tendency to hang together of the 
characters that go in together. We must now take up 
this question. It is one of the most modern develop- 
ments of the Mendelian theory — one that at first 
seemed to throw doubt on the fundamental idea of 
random assortment that gives Mendel’s proportion 
9:3:3:1. But I believe we can now offer a reasonable 
explanation, which shows that we have to do here with 
an extension of Mendelism that in no sense invalidates 
Mendel’s principle of segregation. It not only extends 
that principle, as I have said, but gives us an oppor- 
tunity to analyze the constitution of the germ-plasm 
in a way scarcely dreamed of two or three years ago. 


92 HEREDITY AND SEX 


The actual numbers obtained in the GR by YW 
cross are as follows. These are the figures that Dexter 


has obtained : 


GRE GR GW és YR¢ YWs 
6080 2870 36 34 2373 


The apparent discrepancy between the expected 
and the realized ratios is due to the linkage of the factors 
that went into the cross. For instance, the factors 
for gray and red that went in with one chromosome are 
linked; likewise their allelomorphs, yellow and white. 
As shown by the analysis, the Ff; female offspring 
will have two sex chromosomes, one of each sort — 
one from the father, the other from the mother. But 
the male will have but one sex chromosome derived 
from the mother. 

If in the germ-cells of the F, females there were 
random assortment of the allelomorphs in the sex 
chromosomes, the ideal ratio of 4:1: 1:1:1 would, as 
has been said, be realized. But if the red and gray 
factors tend to remain together since they go in 
together in the one chromosome, and the yellow and 
white in the other chromosome tend to remain together, | 
and if the chances are about 84 to 1 that the factors 
that enter together remain together, the realized ratio 
of 170: 84:1: 1:84 will be found. 

Experiments show that, for these two factors, the 
chances are about 84 to | that the factors that go in 
together remain together; hence the departure from 
Mendel’s ratios for these two pairs of characters. We 
may make a general statement or hypothesis that 
covers cases like these, and in fact all cases where 


THE MENDELIAN PRINCIPLES OF HEREDITY 93 


linkage occurs: viz. that when factors lie in different 
chromosomes they freely assort and give the Mendelian 
expectation ; but when factors lie in the same chromo- 
some, they may be said to be linked and they give 
departures from the Mendelian ratios. The extent to 
which they depart from expectation will vary with 
different factors. Ihave suggested that the departures 
may be interpreted as the distance between the factors 
in question. 


A THEORY OF LINKAGE 


In order to understand more fully what is meant 
by linkage on the interpretation that I have here 
followed, it will be necessary to consider certain changes 
that take place in synapsis. The sex chromosomes 








FA Janssens, ad rat. del Imp. L, Meveset, Brum. S Singer, hich. 


Fic. 49. — Illustrating chiasma-type theory. 1 and 2, from Triton 
cristatus, 3-46, chromosomes of Batracoseps attenuatus. Note especially 
the chiasma shown in 13. (After Janssens). 


94 HEREDITY AND SEX 


(when two are present as in the female), like all other 
chromosomes, unite in pairs at the synaptic period. 
A recognized method of uniting is for like chromosomes 
to come to lie side by side. 

Before they separate, as they do at one of the two 
maturation divisions, each chromosome may be seen 
to be split throughout the length. Thus there are 





Faw 


A ’ 
=< 
x 


sy 





mage, . 


fig 38. ee Ww) Moss 





y 
eet 


Fia. 50. — Chromatin filaments in the amphitene stage from spermato- 
cytes of Batracoseps. (After Janssens.) 


formed four parallel strands each equivalent to a 
chromosome — the tetrad group. At this time Jans- 
sens has found that cross unions between the strands 
are sometimes present (Fig. 49). In consequence 
a strand is made up of a part of one chromosome and 
a part of another. Whether this cross union can be 
referred to an earlier stage — at the time when the two 
like chromosomes come together, when they can be 


THE MENDELIAN PRINCIPLES OF HEREDITY 95 


seen to twist around each other (Fig. 50) —1is not 
certain; but the fact of the existence of cross connec- 
tions is the important point. A consequence of this 
condition is that the chromosomes that come out of 
the tetrad may represent different combinations of 
those that united to form the group. On the basis 
of this observation we can explain the results of associ- 
ated inheritance. For, to the same extent to which 
the chromosomes that unite remain intact, the factors 
are linked, and to the extent to which crossings occur 
the exchange of factors takes place. On the basis 
of the assumption of the linear arrangement of the 
factors in the chromosomes the distance apart of the 
factors 1s a matter of importance. If two factors 
lie near together, the chance of a break occurring be- 
tween them is small in proportion to their nearness. 
We have found that some factors cross over not once 
in a hundred times. I interpret this to mean that they 
lie very near together in the chromosome. 

Other factors cross over to various degrees; in the 
extreme cases the chance is one to one that they cross 
over. In such cases the factors he far apart — perhaps 
near the ends of the chromosome. 

The strongest evidence in favor of this view is found 
when the constant relation of the factors to each other 
is considered. If, for instance, we know the distance 
from A to B (calculated on the basis of crossing over) 
and from B to C, we can predict what A and C will do 
when they are brought into the hybrid from two 
parents. If a fourth factor, D, is discovered and its 
distance from A is made out, we can predict before the 
experiment is made what will take place when D and 


96 HEREDITY AND SEX 


B or D and C are combined. The prediction has been 
fulfilled so many times and in so many ways that we 
feel some assurance that we have discovered here a 
working hypothesis of considerable interest. If the 
hypothesis becomes established, it will enable us to 
analyze the structure of the chromosomes themselves 
in the sense that we can determine the relative location 
of factors in the chromosomes. If, as seems not 
improbable, the chromosomes are the most important 
element in Mendelian inheritance, the determination 
of the linear series of factors contained in them becomes 
a matter of great theoretical interest; for we gain 
further insight into the composition of the material 
on which heredity itself depends. 

There is a corollary to this explanation of crossing 
over that has a very direct bearing on the results. In 
the male there is only one sex chromosome present. 
Hence crossing over is impossible. The experimental 
results show that no crossing over takes place for 
sex-linked factors in the male of drosophila. 

Other factors, however, lie in other chromosomes. 
In these cases the chromosomes exist in pairs in the 
male as well as in the female. Does crossing over 
occur here in both sexes? Let me illustrate this by 
an example. In drosophila the factor for black 
body color and the factor that gives vestigial wings 
lie in the same chromosome, which we may call the 
second chromosome. If a black, long-winged female 
is crossed with a gray vestigial male, all the offspring 
will be gray in color and have long wings, since these 
are the dominant characters. If these F, flies are 
inbred, the following classes will appear: 


THE MENDELIAN PRINCIPLES OF HEREDITY of, aa 
E 1 @ VV 1 
Gray Long Black Long Gray Vestigial 

2316 1146 (ad 


It will be noted that there are no black vestigial 
flies. Their absence can be explained on the assump- 
tion that no crossing over in the male, between the 
factors in the second chromosome, has taken place. 

But if another generation (/;) is raised, some black 
vestigial flies appear. With these, it is possible to test 
the hypothesis just stated. If, for instance, some of 
the long, gray, Ff, females are mated to black vestigial 
males, the following classes are produced : 


GL BL GV eee y 23: 
578 1413 jaaye 307 


The results are explicable on the view that crossing 
over takes place in the germ-cell of the F, female, 
and that the chance that such will occur is as 1 to 3. 

But if the long-winged, gray, /; males are crossed 
to black vestigial females, only the following classes 
are produced : 


/ BL GV 
992 721 


These results are in accord with the hypothesis that 
no crossing over takes place between the second 
chromosomes in the Ff; male. Why crossing over 
should occur. in the F,; female, and not in the F; 
male, we do not know at present; and until the 
synaptic stages in the males and females have been 
carefully studied, we must wait for an answer to the 
question. 


98 HEREDITY AND SEX 


THREE SEX—LINKED FACTORS 


When three sex-linked factors exist in the same 
chromosomes, we have a method by means of which 
the ‘‘ crossing-over’’ hypothesis may be put to a further 
test. Sturtevant has recently worked over the evidence 











E/ 8  G/ H/ 


Fig. 51.— A-D, YW and GR that enter (A), crossing over to give YR 
and GW asseeninD. EH-E;,, no crossing over. F-—F;, crossing between WM . 
and RL. G—Gi, crossing between YW andGR. H-—A,, double crossing over 
of YWM and GRL, to give YRM and GWL. 


for a case of this kind. He analyzed the data of the 
cross between a fly having gray color, red eyes, long 
wings, mated to a fly with yellow color, white eyes, 
and miniature wings. The relative location of these 
three factors is shown in the above diagram (Fig. 51, 


THE MENDELIAN PRINCIPLES OF HEREDITY 99 


E, F, G, H). The F;, flies gave the expected re- 
sults. These inbred gave the following F. significant 
classes :! 


lL v | 
GRL YWM GWM YRL GRM YWL GWL YRM 
2089 1361 17 23 eteih | telly 5 0 


In these results the classes where single crossing over 
is shown are GWM (17) and YRL (28) (Fig. 51, G, 
G’) and GRM (887) and YWL (817) (Fig. 51, F, F’). 

There are two classes, namely, GWL (5) and YRM 
(0) (Fig. 51, H, H’), which involve double crossing over. 
In order that they may take place, the two sex chromo- 
somes in the female must break twice and reunite 
between the factors involved, as shown in the diagram. 
Such a redistribution of the parts of the homologous 
chromosomes would be expected to occur rarely, and 
the small number of double crossovers recorded in 
the results is in accord with this expectation. 

In these questions of linkage we have considered 
some of the most recent and difficult questions in the 
modern study of heredity. We owe to Bateson and 
his collaborators the discovery of this new departure. 
In plants they have recorded several cases of linkage, 
and other authors, notably Correns, Baur, Emerson, 
East, and Trow have described further cases of the 
same kind. Bateson has offered an interpretation 
that is quite different from the one that I have here 
followed. His: view rests on the assumption that 
separation of factors may take place at different times, 
or periods, in the development of the germinal tissues. 


1 The classes omitted are those that do not bear on the question 
in hand.. 


100 HEREDITY AND SEX 


In a word, he assumes that assortment is not confined 
to the final stages in the ripening of the germ-cells, 
but may take place at any time in the germ-tract. 
It seems to me, however, if the results can be brought 
into line with the known changes that take place 
in the germ-cells at the time when the maternal and 
paternal chromosome unite, that we have not only 
a simpler method of dealing with these questions, 
but it is one that rests on a mechanism that can be 
studied by actual observation. Moreover, on purely 
a priorz grounds the assumptions that I have made seem 
much simpler and more tangible than the assumptions 
of “‘reduplication’”’ to which Bateson resorts. 

But leaving these more theoretical matters aside, 
the evidence from a study of sex-linked characters shows 
in the clearest manner that they, while following Men- 
del’s principle of segregation, are also undeniably asso- 
ciated with the mechanism of sex. There is little 
doubt that sex itself is inherited in much the same 
way, since we can explain both in terms of the same 
mechanism. ‘This mechanism is the behavior of the 
chromosomes at the time of the formation of the germ- 
cells. 


CHAPTER IV 


SECONDARY SEXUAL CHARACTERS AND THEIR RELA- 
TION TO DARWIN’S THEORY OF SEXUAL SELECTION 


IN his ‘‘ Origin of Species”? Darwin has defined Sexual 
Selection as depending ‘‘on a struggle between the 
individuals of one sex, generally the males, for the 
possession of the other sex. The result is not death 
to the unsuccessful competitor, but few or no offspring. 
Sexual selection is, therefore, less rigorous than natural 
selection. Generally, the most vigorous males, those 
which are best fitted for their places in nature, will 
leave most progeny. But in many+*cases, victory 
depends not so much on general vigor, as on having 
special weapons, confined to the male sex. <A hornless 
stag or spurless cock would have a poor chance of leav- 
ing numerous offspring. Sexual selection, by always 
allowing the victor to breed, might surely give indomi- 
table courage, length to the spur, and strength to the 
wing to strike in the spurred leg, in nearly the same 
manner as does the brutal cock-fighter by the careful 
selection of his best cocks.”’ 

Darwin continues: ‘‘Amongst birds, the contest 
is often of a more peaceful character. All those who 
have attended to the subject, believe that there is the 
severest rivalry between the males of many species 
to attract, by singing, the females. The rock-thrush 
of Guiana, birds of paradise, and some others, con- 

101 


102 HEREDITY AND SEX 


gregate, and successive males display, with the most 
elaborate care, and show off in the best manner, their 
gorgeous plumage; they likewise perform strange 
antics before the females, which, standing by as spec- 
tators, at last choose the most attractive partner.” 

Here we have two different pictures, each of which 
attempts to give an account of how certain differences 
between the sexes have arisen — differences that we 
call ‘‘secondary sexual characters.” 

On the one hand we deal with a contest between 
the males; on the other with choice by the female. 
The modus operandi is also different. After battle 
the successful male takes his pick of the females. If 
the scheme is to work, he must choose one that will 
leave the most offspring. 

On the other hand, we have the tourney of love. 
The males ‘‘show off’’?; the females stand by spell- 
bound and ‘‘at last choose the most attractive partner.” 

Now, concerning this display of the males, I beg 
leave to quote a paragraph from Wallace’s ‘‘ Natural 
Selection and Tropical Nature” : 

‘“‘Tt is a well-known fact that when male birds possess 
any unusual ornaments, they take such positions or 
perform such evolutions as to exhibit them to the best 
advantage while endeavoring to attract or charm 
the females, or in rivalry with other males. It is 
therefore probable that the wonderfully varied decora- 
tions of humming-birds, whether burnished breast- 
shields, resplendent tail, crested head, or glittering 
back, are thus exhibited ; but almost the only actual ob- 
servation of this kind is that of Mr. Belt, who describes 
how two males of the Florisuga mellwora displayed 


SECONDARY SEXUAL CHARACTERS 103 


their ornaments before a female bird. One would 
shoot up like a rocket, then, suddenly expanding the 
snow-white tail like an inverted parachute, slowly 
descend in front of her, turning around gradually to 
show off both back and front. The expanded white 
tail covered more space than all the rest of the bird, and 
was evidently the grand feature of the performance. 
Whilst one was descending the other would shoot 
up and come slowly down expanded.”’ 

There is just a suspicion in my mind that these males 
were otherwise engaged, for while I know nothing 
about the habits of these humming birds I find on the 
next page of “ Tropical Nature” this statement: 

“Mr. Gosse also remarks: ‘All the humming- 
birds have more or less the habit, when in flight, of 
pausing in the air and throwing the body and tail into 
rapid and odd contortions. This is most observable 
in Polytmus, from the effect that such motions have 
on the long feathers of the tail. That the object of 
these quick turns is the capture of insects, I am sure, 
having watched one thus engaged.’ ”’ 

If what I have just said implies that I take a lght- 
hearted or even facetious attitude toward Darwin’s 
theory, I trust that my position will not be misunder- 
stood. Darwin brought together in his book on the 
‘Descent of Man”’ a mass of interesting observations 
for which he suggested a new theory. No one can 
read his wonderful book without the keenest interest, 
or leave it without high admiration for the thorough- 
ness with which the subject is treated ; for the ingenuity 
and skill with which the theory is applied to the facts, 
and, above all, admiration for the moderation, modesty, 


104 | HEREDITY AND SEX 


and honesty with which objections to the theory are 
considered. 

I will let no one admire Darwin more than I admire 
Darwin. But while affection and respect and honor 
are the finest fruits of our relation to each other, we 
cannot let our admiration for the man and an ever 
ready recognition of what he has done for you and for 
me prejudice us one whit in favor of any scientific 
theory that he proposes. For in Science there is no 
authority ! We should of course give serious considera- 
tion to any theory proposed by a man of such wide expe- 
rience and trained judgment as Darwin ; but he himself, 
who broke all the traditions of his race, would be the 
first to disclaim the value of evidence accepted on 
authority. 


From the definition of sexual selection with which 
we started it may be said that Competition and Courtship 
stand for the two ways in which Darwin supposes 
the secondary sexual characters to have arisen. 

Competition amongst the males is only a form of nat- 
ural selection, as Darwin himself recognized (if we leave 
out of account the further assumption that the victor 
chooses his spoils). We may dismiss this side of the 
problem as belonging to the larger field of natural selec- 
tion, and give our attention mainly to those secondary 
sexual characters that Darwin supposes to have arisen 
by the female choosing the more ornamented suitor. 

I shall first bring forward some of the more striking 
examples of secondary sexual characters in the animal 
kingdom. These characters are confined almost ex- 
clusively to three great groups of animals — Insects, 


SECONDARY SEXUAL CHARACTERS 105 


Spiders, and Vertebrates. There are a few scattered 
instances found in other groups, but they are rare. 
In the lowest groups they are entirely absent, and are 





Fig. 52. — Four species of beetles in which the male (to the left) has horns 
which are absent in the female (to the right). (After Darwin.) 


not found at all in plants; or rather, if character- 
istic differences exist in plants, they are not called by 
this name — for plants cannot see or move and there- 
fore cannot court each other. 


106 HEREDITY AND SEX 


In fact, sight in the sense of forming visual pictures 
can occur only when eyes are well developed. This 





Fic. 53. — Male (to left) with long eye stalks and female (to right) of 
a fly, Achia longividens. (After Wood.) 
may be taken to score a point in favor of Darwin’s 
hypothesis. 

In the group of insects the most noticeable differences 
occur in the butterflies and moths, and in flies. A 
few cases are found in the beetles and bugs. The 
male cicada’s shrill call is supposed to attract the 





Fia. 54. — Male to left with horns and female to right without horns of a 
fly, Elaphomyia. (After Wood.) 
females. The males of certain beetles have horns — 
the female lacks them (Fig. 52). 
In a genus of flies the eyes are stalked, and the 


SECONDARY SEXUAL CHARACTERS 107 


eyes of the male have stalks longer than those of 
the female (Fig. 53). In another genus of flies there 
are horns on the head like the antlers of the stag 
(Fig. 54). 

In the spiders the adult males are sometimes very 
small in comparison with the females (Fig. 55). The 
size difference may be regarded as a secondary sexual 





Fic. 55. — Male (to left) and female (to right) of a spider, Argiope aurelia. 
(From ‘‘ Cambridge Natural History.’’) 


character. Darwin points out, since the male is some- 
times devoured by the female (if his attentions are 
not desired), that his small size may be an adaptation 
in order that he may more readily escape. But the 
point may be raised as to whether he is small in order 
to escape; or whether he is eaten because he is small. 

In one of our native spiders, Habrocestum splendida, 
the adult males and females are conspicuously different 


108 HEREDITY AND SEX 


in color —the male more highly colored than the 
female. In another native species, Maema vittata, 
there are two kinds of males, both colored differently 
from the female. 

Passing over the groups of fishes and reptiles in 
which some striking cases of differences between the 
sexes occur, we come to the birds, where we find the 
best examples of secondary sexual characters. 





Fia. 56. — Superb bird of paradise. 
(After Elliot.) 

In the white-booted humming bird (Fig. 14) two 
of the tail feathers of the male are drawn out, their 
shafts denuded of the vanes except at the tip where 
the feather ends in a broad expansion. 

In the great bird of paradise, of the Aru Islands (Fig. 
13), the male has wonderful plumes arising from the 
sides that can be erected to produce a gorgeous display. 


SECONDARY SEXUAL CHARACTERS 109 


The female is modestly clothed. In the male of the 
superb bird of paradise (Fig. 56), the mantle behind 
the neck, when erected, forms a striking ornament ; 
and on the breast there is a brilliant metallic shield. 

In the six-shafted bird of paradise (Fig. 57) the 
male has on its head six feathers with wiry shafts, 





Fia. 57. — Six-shafted bird of paradise. 
(After Elliot.) 


ornaments that occur in no other birds. In the king 
bird of paradise there are remarkable fans at the 
sides of the body of the male that can be expanded. 
The feathers of the fan are emerald-tipped. The 
two middle feathers of the tail are drawn out into 
‘“‘wires’”’ with a green web at one side of the tip. 

In mammals, secondary sexual differences are very 


110 HEREDITY AND SEX 


common, although startling differences in color are 
rather rare. In the male the coat of fur is often darker 
than that of the female. 

In many deer the antlers are present in the male 
alone. In Steller’s sea-lion the male is much larger 
and stronger than the female. In a race of the Asiatic 
elephant the male has tusks much larger than those 
of the female. 

If we fix our attention exclusively on these remarkable 


—" 


ete res 





Fig. 58. — Wilson’s phalarope, female (in center), male (to right and 
behind). A bird in winter plumage is at the left. (From Eaton, “ Birds of 
New York.’’) 


cases where differences between the sexes exist, we 
get a one-sided impression of the development of 
ornamentation and color differences in animals. We 
must not forget that in many cases males and females 
are both highly colored and exactly alike. We forget 
the parrots, the cockatoos, the kingfishers, the crowned 
pigeons, toucans, lories, and some of the starlings; 
the ‘‘briliant todies” and the ‘‘sluggish jacamars” - 
whose brilliant metallic golden-green breasts rival 


SECONDARY SEXUAL CHARACTERS Ba 


those of the humming-birds; we forget the zebras, 
the leopards; the iridescent interiors of the shells of 
many mollusks; the bright reds and purples of starfish, 
worms, corals, sea anemones, the red, yellow, and green 
sponges, and the kaleidoscopic effect of the microscopic 
radiolarians ; — a brilliant array of color. 





Fig. 59. — (A), female of a copepod, Calocalanus plumosus. (B), a female 
of Calocalanus parvus. (C) male of last. 


In the egret both males and females have remark- 
able nuptial plumes, which, had they been present in 
one sex alone, would have been classified as secondary 
sexual characters. It does not appear that selection 
had anything to do with their creation. 

Our common screech owl exists in two colored types 
sharply separated. No one is likely to ascribe these 
differences to sexual selection, yet if one sex had been 


112 HEREDITY AND SEX 


red and the other gray, the difference would have been 
put down to such selection. There are also cases like 
the phalarope, shown in Fig. 58, where the female is 
more highly ornamented than the male. In fact, for 
these cases, Darwin supposed that the males select 
the females; and in support of this view he points out 
that the females are more active, while the male con- 
cerns himself with the brooding of the eggs. In some 
of the marine copepods female ornamentation is car- 
ried to even a higher point. In Calocalanus plumosus 
the female has one of the tail setze drawn out into a long 
feather-like structure (Fig. 59). In another species, 
C. parva, all eight sete of the tail of the female are 
feather-like (Fig. 59, B), while the male (Fig. 59, C) 
lacks entirely these ‘‘ornaments.”’ 

In some butterflies also, two, three, or more types of 
females are known, but only one male type. I shall 
have occasion later to consider this case. 


COURTSHIP 


The theory of sexual selection hinges in the first 
place on whether the female chooses amongst her 
suitors. 

It has been objected that the theory is anthropo- 
morphic — it ascribes to beetles, butterflies, and birds 
the highly developed esthetic sense of man. It has 
been objected that the theory leaves unexplained the 
development of this esthetic sense itself, for unless the 
female kept in advance of the male it is not self-evident 
why she should go on selecting the more highly orna- 
mented. If she has advanced esthetically, what has 
brought it about? In answer to this last question 


SECONDARY SEXUAL CHARACTERS 113 


Allen suggests that if the word conspicuousness is sub- 
stituted for the word beauty, the objection may to some 
extent be met. The more conspicuous male would be 
more likely to attract attention and be selected. 

It has been pointed out that there is more than a 
suspicion that the contests of the males for the females 
are sham affairs. ‘They are like certain duels. There 
is seldom any one hurt. There are very few records of 
injured males, but many accounts of tremendous 
battles. And he who fights and runs away will live 
to mate another day. 

It is clear, I think, that the case against the theory 
must rest its claims on actual evidence rather than on ar- 
guments or poetry pro or con. Darwin admitted that 
the evidence was meager. Since his time something 
more has been done. Let us consider some of this new 
evidence. 

It will be conceded, I think, that Alfred Wallace, 
through his wide experience with animals in their 
native haunts, is in a position to give weighty evidence 
concerning the behavior of animals. He was with 
Darwin a co-discoverer of the theory of Natural Se- 
lection and cannot be supposed to be prejudiced against 
the selection principle. Yet Wallace has from the 
beginning strongly opposed the theory of sexual se- 
lection. Let me quote him: 

Referring to Darwin’s theory of Sexual Selection — 

‘“‘T have long held this portion of Darwin’s theory to 
be erroneous — and have held that the primary cause 
of sexual diversity of color was the need of protection, 
repressing in the female those bright colors which are 
normally produced in both sexes by general laws.”’ 


114 HEREDITY AND SEX 


Again, Wallace says: ‘‘To conscious sexual selec- 
tion — that is, the actual choice by the females of the 
more brilliantly colored males or the rejection of those 
less gaily colored —I believe very little if any effect 
is directly due. It is undoubtedly proved that in 
birds the females do sometimes exert a choice; but 
the evidence of this fact, collected by Mr. Darwin 
(‘Descent of Man,’ chap. xiv), does not prove that color 
determines that choice, while much of the strongest 
evidence is directly opposed to this view.”’ 

Again, Wallace says: ‘‘Amid the copious mass of 
facts and opinions collected by Mr. Darwin as to the 
display of color and ornaments by the male birds, there 
is a total absence of any evidence that the females, as 
a rule, admire or even notice this display. The hen, 
the turkey, and the peafowl go on feeding, while the 
male is displaying his finery; and there is reason to 
believe that it is his persistency and energy rather than 
his beauty which wins the day.”’ 

Hudson, who has studied the habits of birds in the 
field, asks some very pertinent questions in connec- 
tion with their performances of different kinds. ‘‘ What 
relation to the passion of love and to the business of 
courtship have these dancing and vocal performances 
in nine cases out of ten? In such eases, for instance, 
as that of the scissor-tail tyrant-bird, and its pyro- 
technic displays, when a number of couples leave their 
nests containing eggs and young to join in a wild aérial 
dance; the mad exhibition of grouped wings; the 
triplet dances of the spur-winged lapwing, to perform 
which two birds already mated are compelled to eall 
in a third bird to complete the set; the harmonious 


SECONDARY SEXUAL CHARACTERS 115 


duets of the oven-birds and the duets and choruses of 
nearly all the wood-hewers, and the wing-slapping 
aérial displays of the whistling widgeons, — will it be 
seriously contended that the female of this species 
makes choice of the male able to administer the most 
vigorous and artistic slaps ?”’ 

He continues: ‘‘How unfair the argument. is, 
based on these carefully selected cases, gathered from 
all regions of the globe, and often not properly reported, 
is seen when we turn to the book of nature and closely 
consider the habits and actions of all the species in- 
habiting any one district.’? Hudson concludes that he 
is convinced that anybody who will note the actions of 
animals for himself will reach the conviction, that 
‘“conscious sexual selection on the part of the female 
is not the cause of music and dancing performances in 
birds, nor of the brighter colors and ornaments that 
distinguish the male.” 

In the spiders Mr. and Mrs. Peckham have described 
in detail the courtship of the males. They believe 
that his antics are specifically intended to attract the 
female. They point out that his contortions are of 
such a sort that his brightest spots are turned toward 
the female. But, as he makes in any case a hundred 
twists and turns, there is some danger of misinterpret- 
ing his poses. Montgomery, who has studied spiders 
of other groups, reaches the conclusion that here the 
‘male is contorted through fear of the female. The male 
goes through some of the same turns if approached by 
another male. The courtship of the male spider is, 
he thinks, a motley of fear, desire, and general 
excitement. 


116 HEREDITY AND SEX 


The evidence that the Peckhams have given, even if « 
taken to mean that the motions of the male attract 
the attention of the female, — and I can see no reason 
why this may not be the case, —fails nevertheless to show 
that the female selects, when she has a chance, the more 
highly colored male. 

Mayer, and Mayer and Soule have made many ex- 
periments with moths. The moth promethea, Callo- 








Fic. 60. — Above, Callosamia promethia, male to left, female to right. 
Below Porthetria dispar, male to left, female to right. 


samia promethea, is distinetly sexually dimorphic, as 
shown in Fig. 60. Mayer’s experiments show that the 
male finds the female entirely by the sense of smell. 
The wings of some 300 males were painted with scarlet 
or green. ‘They mated as often as did the normal male 
with which they competed. 

Where the wings of males were stuck on the female 
in place of her own wings, no disturbance in the mating 
was observed. Conversely, normal females accepted 


SECONDARY SEXUAL CHARACTERS 117 


males with female wings as readily as they accepted 
normal males. 

In the gipsy moth (Porthetria dispar), the male is 
brown and the female white (Fig. 60). Here again 
it was found that the males are guided solely by the 
odor of the female. 

The silkworm moth is also sexually dimorphic. Kel- 
logg has shown that males with blackened eyes find a 
female with as much precision as does a moth with 
normal eyes. 

If the antenne are cut off, however, the male can not 
find the female unless by accident he touches her. He 
then mates. The female has scent glands whose odor 
excites the male with normal antenne even at some dis- 
tance. Chemotaxis and contact are the active agents 
in mating. ‘The eyes do little or nothing. 

Andrews has found that touch determines mating in 
the crayfish. Pearse has obtained similar results. 
Chidester has shown the same thing for crabs. Holmes 
found this kind of behavior in Amphipoda. Fielde and 
Wheeler have also found that in ants sex-discrimination 
is through smell or by what Forel calls contact-odors. 

Montgomery and Porter recognize touch as the most 
important factor in mating in spiders. Petrunke- 
witsch has shown that in the hunting spider vision also 
helps the sexes to find each other. Tower has found 
that contact or odor rather than sight is the important 
condition in mating in leptinotarsa. 

I am able to give the unpublished results of A. H. 
Sturtevant on the mating of the fruit fly, drosophila. 
The male carries on an elaborate courtship in the 
sense that he circles around the female, throws out one 


¢ 


118 HEREDITY AND SEX 


wing, then the other, and shows other signs of excite- 
ment. The male has sex combs on his fore legs, the 
female lacks them. Lutz cut them off and gave the 
female a choice between such a male and a normal 
male. One was chosen as often as the other. The 
wings of the male and female are wonderfully irides- 
cent. Sturtevant cut off the wings of a male and 
matched him against a normal male. The female 
showed no marked preference. The converse experi- 
ment, when a clipped female competed with a normal 
female, showed no selection on the part of the males. 

If instead of allowing two males (a normal and a 
clipped) to compete for one female, a female is given to 
each male separately, and the interval before mating is 
noted, it is found that on an average this interval is 18 
minutes for the normal and 40 minutes for the clipped. If 
any such difference existed in the first case, when the two 
males were competing, we should expect a much greater 
selection in favor of the normal male than was actually 
found. ‘This would seem to mean that the female is 
more quickly aroused by the normal male, and hence 
when both males are present she will accept the clipped 
male more quickly than when he alone is present. This 
suggests that normal courtship precipitates copulation. 

In the following experiménts the female was offered 
a choice between a new type (mutant) with white eyes 
and a normal male. Conversely, the white-eyed fe- 
male had a like alternative. The evidence shows that 
the more vigorous male —the red-eyed male — is 
more successful. 

Since vision itself is here involved, for the white- 
eyed flies are probably partly blind, the observations 


SECONDARY SEXUAL CHARACTERS 119 











RED VERSUS WHITE EYES. GRAY VERSUS YELLOW COLOR. 
Red ee 9 a Ory be 9 nH 
Wiles | vais 9 0c Yellow d {Yas 9 aan 
Red 2 ee e ioe. Sate J es 
White ete : a Yellow ? in fa i a 
Sharer eae VERSUS CLIPPED } GRAY-WHITE ne ave 
WINGS. WHITE. 
Normal ? | Novel 9 6t Se ee 
Normal ¢ { Smet So ag SS aoa 








were repeated with a new type that had yellow wings. 
The gray male is more successful and the yellow females 
less resistant. The results are in accord with the as- 
sumption that greater vigor is an important factor 
in success. | 

The following mating bears on this point. Stur- 
tevant used in competition a red- and a vermilion- 
eyed male. The latter seems as vigorous as is the 
red-eyed type. The results were : 


Red 3 11 
Vermilion 14 


Red @ 


showing that the red-eyed male has no advantage 
when the males are equally vigorous. 

This evidence, taken as a whole, seems to me to show 
with some probability that sight plays a minor réle in 


120 HEREDITY AND SEX 


courtship. It is so inferior to vigor, to the sense of 
smell and to touch in the lower animals at least, that 
it is very questionable whether it has had anything 
more to do with mating than helping the sexes find 
each other. 


VIGOR AND SECONDARY SEXUAL CHARACTERS 


We have seen that Darwin himself has stated ex- 
plicitly that unless the secondary sexual characters 
are associated with greater vigor, or productivity, 
nothing can be accomplished. 

It will be recalled that Wallace, who disbelieved in 
Darwin’s theory of sexual selection, attempted to ac- 
count for the appearance of secondary sexual characters 
on the ground of the greater vigor of the. male (he 
sometimes says vitality and again activity of the male) 
at the breeding season. The vigor is assumed to be 
associated with the development of the sex glands 
‘at this time. This may be admitted, but whether the 
vigor is the result of the sex glands, or the sex glands of 
the vigor, is a nice point that I shall not try to decide. 
It may appear that Wallace’s view is in part justified 
from the facts that we have examined. But I do not 
think so. In the first place, he attempts the impos- 
sible task of explaining the outgrowths and colors that 
appear in special regions by the local activity of the 
muscles (for example) in those regions. The facts 
before us do not support any such interpretation. The 
Peckhams easily overturn his argument, as applied to 
spiders. 

Second, in birds, to which Wallace mainly refers, the 
sex glands of the male do not affect the secondary 


SECONDARY SEXUAL CHARACTERS 121 


sexual characters of the male, while the sex glands of the 
female suppress these characters. 

Wallace’s theory leaves out of account the hereditary 
factor that is also present and which acts quite apart 
from the physiological effects of the sex glands. 

Cunningham, who has more recently written on the 
same subject, accepts the hormone hypothesis as the 
basis for all cases of secondary sexual characters. 
But he fails to make good his view when it is applied 
_ to insects, for reasons that we shall take up later. He is 
especially concerned, however, in the attempt to make 
plausible his own hypothesis that secondary sexual 
characters have arisen through the use of the parts, or 
through special nervous or blood supplies to certain lo- 
calities of the body which become suffused during sexual 
excitement. In both cases he thinks the increased local 
activity will cause the cells to produce hormones that 
will be dispersed throughout the body, and absorbed 
by other cells. The germ-cells will in this way get 
their share and carry over the hormone to the next 
generation. 

Cunningham forgets one important point. If these 
imaginary hormones can get out of cells and into germ- 
cells, they can get out of the germ-cells again. Hence 
in the long period of embryonic and juvenile existence 
through which the individual passes before the second- 
ary sexual characters appear they would surely be lost 
from the body like any other ordinary hormone. 


CONTINUOUS VARIATION AS A BASIS FOR SELECTION 


And now let us turn to an entirely different aspect of 
the matter. What could selection do, admitting that 
selection may take place. For fifty years it has been 


122 HEREDITY AND SEX 



































Fic. 61. — I. Diagram of five pure lines of beans (A, B, C, D, and £) 
and a population formed by their union, A—H. II. Diagrams illustrating a 
pure line of beans and two new biotypes derived from it. The upper 
diagram indicates the original biotype; the second and third diagram in- 
dicate the elongated (narrower) and shorter (broader) type of beans. X 
indicates the average class of the original biotype. (After Johannsen.) 


SECONDARY SEXUAL CHARACTERS 123 


taken for granted that by selecting a particular kind 
of individual the species will move in the direction 
of selection. 

A few examples will bring the matter before us. If 
we take a peck of beans and put all of those of one size 
in one cylinder and those of other sizes in other cyl- 
inders, and place the cylinders in a row, we get a result 
like that in Fig. 61, A—E. If we imagine a line joining 


+36 +26 +6 O +6 +26 +36 

Fic. 62. — The normal binomial curve or the “ideal curve’’ of distribu- 
tion. At the base line, the directions from the average value (0) are 
_ indicated with the standard deviation (7) as unity. (After Johannsen.) 


the tops of the beans, the line gives a curve like that 
shown in Fig. 62. This is known as the curve of prob- 
ability. The curve can be, of course, most readily 
made by making the measurements directly. Most 
individuals of such a population will have the charac- 
ter developed to the degree represented by the highest 
point in the curve. Now if two individuals standing 
at one side (let us say with the character in question 
better developed than the average) become the parent 


124 HEREDITY AND SEX 


of the next generation, their offspring will make a new 
curve that has moved, so to speak, in the direction of 
selection (Fig. 63). 

If again two more extreme individuals are selected, 
another step is taken. The process is asswmed to go 
on as long as the selection process is maintained. 

So the matter stood until a Danish botanist, Johann- 
sen, set seriously to work to test the validity of the 
assumption, using a race of garden beans for his meas- 
urements. He discovered in the first place that popu- 


isis 


Fic. 63. — Schematic representation of the type-shifting effect of selec- 
tion from the point of view of Galton’s reversion theory. The * marks the 
point on the curves of A, Ai, A» from which the selection is supposed to be 
made. (Goldschmidt.) 


lations are made up of a number of races or ‘‘pure 
lines.’”” When we select in such a population we sort 
out and separate its constituent races, and sooner 
or later under favorable conditions can get a pure 
race. Selection has created nothing new; it has 
picked out a particular preéxisting race from a mixed 
population. | 

Johannsen has shown that within a pure line selec- 
tion produces no effect, since the offspring form the 
same group with the same mode as the group from which: 
the parents came. The variability within the pure 
lines is generally ascribed to environmental influences 


SECONDARY SEXUAL CHARACTERS 125 


which are recurrent in each generation. The germ- 
plasm is homogeneous for all members of the pure line, 
while in a mixed population the germ-plasm is not the 
same for all individuals. 

Darwin himself saw this to some extent, for he has 
repeatedly pointed out that selection depends on the 
materials offered to it by variation; that in itself it 
can produce nothing. Yet from Darwin to Johannsen 
the teaching of the post-Darwinians has been such as to 
lead most people to believe that selection is a causative 
or creative principle that will explain the progressive 
development of animals and plants., |. y 


DISCONTINUOUS VARIATION OR MUTATION AS A BASIS 
“FOR SELECTION 


The second great movement since Darwin has been 
to show that hereditary variations do not give a con- 
tinuous series but a discontinuous one. Bateson and 
De Vries brought forward some twelve years ago evi- 
dence, in favor, of this view, that has gone on increasing 
in volume at®a@n#amazing rate. 

I cannot attempt to discuss this evidence here, but 
I may point out the bearing of the new point of view 
on the meaning of secondary sexual characters. 

In a number of butterflies there occur two or three 
or even more different kinds of females. One of the most 
remarkable cases of the kind is that of Papilio polytes 
that lives in India and Ceylon. It has a single male 
type and three types of females (Fig. 64). 

Wallace, who first observed that the three types of 
female belong to one male type, argued that two 
of these three types owe their origin to their resem- 


126 HEREDITY AND SEX 


blance to butterflies of other species that are protected, 
namely, Papilio aristolochia and P. hector. These 









P. potytes, o 


Fia. 64. — Papilio polytes ; male (upper left) and three types of female 
(to right). The ‘‘models,’’ which two of these females are supposed to 
‘“mimic,’”’ are shown to their left. 


two feed on the poisonous plant aristolochia and are 
said to be unpalatable. The two aberrant types of 
P. polytes bearing a close resemblance to these two 


SECONDARY SEXUAL CHARACTERS 127 


species have been dubbed the hector form and the 
aristolochia form. : 

Wallace, and those who adhere to the same view, 
believe that the resemblance of the model and the 
mimic has come about through the accumulation of 
minute variations which have survived as a result of 
their advantages. In a word, the process of natural 
selection is assumed to have gradually brought about 
the evolution of these two new types of females. 

This case has been recently examined by Punnett. 

Punnett says that while in cabinet specimens the 
resemblance between the model and the mimic is re- 
markably close, yet in the living animals, with their. 
wings spread out, the resemblance is less marked, espe- 
cially the resemblance between the hector model and 
the polytes mimic. At a distance of a few yards the 
difference between the two is easily seen. 

When flying the differences are very apparent. ‘‘The 
mode of flight of P. polytes is similar for all three forms, 
and_is totally distinct from that of P. hector and P. 
aristolochia.”’ In flight the latter pursue an even 
course, while the polytes form follow a lumbering 
up and down course. Punnett thinks these differ- 
ences are so distinct that they are ‘‘unlikely to be 
confounded by an enemy with any appreciation of 
color or form.” 

Moreover, in Ceylon at least, the distribution of the 
model and its mimic is very different from what is 
expected on the theory of mimicry. He concludes that 
the facts relative to their distribution ‘‘are far from 
lending support to the view that the polymorphic 
females of P. polytes owe their origin to natural selec- 


128 HEREDITY AND SEX 


tion, in the way that the upholders of the theory of 
mimicry would lead us to suppose.”’ 

After considering the difficulties that the theory of 
mimicry has to contend with, Punnett points out that 
dimorphic and polymorphic species are not uncommon 
in butterflies, and that in many of these cases there can 
be little or no question of mimicry having anything 





Fia. 65.— Papilio turnus ; female (above) and male (below), and the variety 
P. turnus glaucus (above, right) which appears only in the female. 


to do with the matter. It is well known that in Lepidop- 
tera the modified form commonly belongs to the female 
sex. In one case (Abraxas grossulariata) it is known 
that the aberrant female type appears sporadically, as a 
sport, and follows Mendel’s law of segregation. Punnett 
shows how the recurrence of the single type of male and 
the three types of females of polytes may also be ac- 
counted for by the recognised methods of Mendelian in- 
heritance. He points out that by the assumption that 


SECONDARY SEXUAL CHARACTERS 129 


these types have suddenly appeared as mutants many 
of the difficulties of the older theories are avoided, 
and that such an assumption is in harmony with 
an ever increasing body of evidence concerning 
variation and heredity. On this view ‘‘natural se- 
lection” plays no part in the formation of these 
polymorphic forms,” nor does sexual selection. The 
absence of transitional forms is explicable on this 





Fia. 66. — Colias philodice, showing two female forms above and 
one male form below. 


view, and unaccountable on the other theory. In 
fact polymorphic forms, if they appear, would be 
expected to persist if they are not harmful to the 
species. 

We have in this country several species of butter- 
flies in which polymorphism exists. In the north 
the species Papilio turnus (Fig. 65) is alike in the male 
and in the female. But in the south two types of 
females exist — one like the male and the other a 
black type. 


130 HEREDITY AND SEX 


In the Eastern States there is a butterfly, Colas 
philodice, in which two types of female exist (Fig. 66). 
Gerould has studied the mode of inheritance of these 
two types and finds that they conform to a scheme in 
which the two females differ by a single factor. The evi- 
dence is strongly in favor of the view that one of these 
forms has arisen as a mutation. There is no need to 
suppose that sexual selection has had anything to do 
with its origin, and no evidence that it owes its exist- 
ence to mimicry of any other species. 

Finally, I should like to speak of a case that has come 
under my own observation. One of the mutants that 
appeared in a culture of drosophila had a new eye 
color that was called eosin. In the female the eye is 
much deeper in color than in the male. The race main- 
tains itself as a bicolor type without any selection. 


CONCLUSIONS 


In conclusion let me try to bring together the main 
considerations that seem to me to throw serious doubts 
on Darwin’s theory of sexual selection. 

First. Its fundamental assumption that the evolution 
of these characters has come about through the “ will,” 
‘“‘choice,”’ or selection of the female is questionable, 
because of want of evidence to show that the females 
make their choice of mates on this basis. There is also 
some positive evidence to show that other conditions 
than selection of the more ornamented individual 
(because he is the more CaO En EE are responsible 
for the mating. 

Second. We have come to have a different concep- 
tion of what selection can do than the sliding scale 


SECONDARY SEXUAL CHARACTERS 131 


assumption that has been current, at least by implica- 
tion, in much of the post-Darwinian writings. 

Third. Recent advances in the study of variations 
have given us a new point of view concerning the na- 
ture of variation and the origin of variations. If we 
are justified in applying this new view to secondary 
sexual characters, the problem appears greatly sim- 
plified. 


CHAPTER V 


THE EFFECTS OF CASTRATION AND OF ‘TRANSPLAN- 
TATION ON THE SECONDARY SEXUAL CHARACTERS 


In several of the preceding chapters I have spoken in 
some detail of seax-linked inheritance. In sex-linked 
inheritance we deal with a class of characters that are 
transmitted to one sex alone in certain combinations, 
and have for this reason often been called sex-limited 
characters; but these same characters can be trans- 
ferred by other combinations, as we have seen, to the 
other sex, and are therefore not sex-limited. 

In contrast to these characters secondary sexual char- 
acters appear in one sex only and are not transferable 
to the other sex without an operation. For instance, 
the horns of the stag and the colors and structures of 
certain male birds are in nature associated with one 
sex alone. 

It has long been recognized in mammals and birds 
that there is a close connection between sexual maturity 
and the full development of the secondary sexual char- 
acters. This relation suggests some intimate correla- 
tion between the two. It has been shown, in fact, in 
some mammals at least, that the development of the 
secondary sexual characters does not take place, or 
that they develop imperfectly, if the sex glands are 
removed. It may appear, therefore, that we are deal- 
ing here with a purely physiological process, any an 

132 


THE EFFECTS OF CASTRATION 133 


the development of these structures and colors is a by- 
product of sex itself, and calls for no further explana- 
tion. 

But the question cannot be so hastily dismissed. 
This can best be shown by taking up at once the ma- 
terial at hand. 


OPERATIONS ON MAMMALS 


In the deer, the facts are very simple. If the very 
young male is castrated before the knobs of the antlers 
have appeared, the antlers never develop. 





Fiq. 67. — Merino; male (horned) and female (hornless). 


If the operation is performed at the time when the 
antlers have already begun to develop, incomplete 
development takes place. The antlers remain covered 
by the velvet and are never thrown off. They are called 
peruke antlers. If the adult stag is castrated when 
the horns are fully developed, they are precociously 


134 HEREDITY AND SEX 


dropped, and are replaced, if at all, by imperfect ant- 
lers, and these are never renewed. 

These facts make it clear that there is an intimate 
relation between the orderly sequence of development 
of the horns in the deer and the presence of the male 
sexual glands. 

In the case of sheep, the evidence is more explicit. 
Here we have carefully planned experiments in which 
both sexes have been studied; and there are breeding 





Fic. 68. — Dorset; male (horned) and female (horned). 


experiments also, in which the heredity of horns has 
been examined. 

In some breeds of sheep, as in the Merinos and 
Herdwicks, horns are present in the males, absent in 
the females (Fig. 67). In other breeds of sheep, as 
in Dorsets, both males and females have horns (Fig. 
68). In still other breeds both sexes lack horns, as 
in some of the fat-tailed sheep of Africa and Asia 
(Fig. 69). 

Marshall has made experiments with Herdwicks — 
a race of sheep in which the rams have large, coiled 
horns and the ewes are hornless. Three young rams 
(3, 4, and 5 months old) were castrated. The horns 
had begun to grow (3, 414, and 6 inches long) at the 
time of operating. They ceased to grow after the 
operation. 


THE EFFECTS OF CASTRATION 135 


A similar operation was also carried out on females. 
Three Herdwick ewe lambs (about 3 months old) were 
operated upon. After ovariotomy, the animals were 
kept for 17 months, but no horns appeared, although 
in one, small scurs developed, in the other two scarcely 
even these. It is clear that the removal of the ovaries 
does not lead to the development of horns lke those 
in the male. | 

Now, the interpretation of this case can be made 
only when taken in connection with experiments in 
heredity. There is a crucial experiment that bears on 
this question. Arkell found when a Merino ewe (a race 
with horned males and hornless females) was bred to a 
ram of a hornless breed, that the sons had horns. In 
this case the factor for horns must have come from the 
hornless mother, while the development of the horns was 
made possible by the presence of the male glands. It 
is evident therefore in the castration experiment that 
a factor for horns is inherited by both sexes, but in order 
that the horns may develop fully, the male glands must 
be present and functional. 

In the Dorset, both sexes are horned, the horns of the 
females are lighter and smaller than the horns of the 
ram (Fig. 68). In the castrated males the horns are 
like those of the females. In this case we must sup- 
pose that the hereditary factor for horns suffices to 
carry them to the point in development reached by the 
females. To carry them further the presence of the sex 
glands of the male is necessary. 

In the case of the hornless breeds I do not know of 
any evidence from castration or ovariotomy. We may 
suppose, either that the factor for horns is absent; or, 


136 HEREDITY AND SEX 


if present, some inhibitory factor must bring about sup- 
pression of the horns. The former assumption seems 
more probable, for, as I shall point out, certain experi- 
ments in heredity indicate that no inhibitor is present 
in hornless breeds. 

The series is completed by cases like that of 
the eland and the reindeer. Both males and females 





Fic. 69. — Fat-tailed hornless sheep (Ovis 
aries steatopyga persicci). 


have well-developed horns. In this case the hereditary 
factors suffice in themselves for the complete develop- 
ment of horns, for even after castration the horns de- 
velop. 

We have anticipated to some extent the conclusions 
arrived at by breeding experiments in these races of 
sheep. The best-known case is that of Wood, whe 
crossed horned Dorsets and hornless Suffolks. As 


THE EFFECTS OF CASTRATION 137 


shown in the picture (Fig. 70) the sons had horns — 
the daughters lacked them. When these are inbred, 
their offspring are of four kinds, horned males, hornless 
males, horned females, hornless females. 

It seems probable that these four classes appear in 
the following proportions : 


Horned 6 Hornless 6 Horned 2  Hornless ? 
os 1 1 As 


The explanation that Bateson and Punnett offer for 
this case is as follows: The germ-cells of the horned race 





Fia. 70. — 1, Suffolk (ram), hornless in both sexes; 2, Dorset (ewe), 
horned in both sexes; 3, Fi ram, horned; 4, #: ewe, hornless; 5-8, the four 
types of F2; 5 and 6 are rams, 7 and 8 are ewes. The hornless rams are 
pure for absence of horns, and the horned ewes are pure for the presence of 
horns. Figs. 5 and 6 represent lambs. (Bateson, after Wood.) 


(both male and female) carry the factor for horns (ff); 
the germ-cells of the hornless race lack the factor for 
horns (h). The female is assumed to be homozygous 
for the sex factor, 7.e. two sex chromosomes (X) are 
present ; while the male has only one sex chromosome 


138 HEREDITY AND SEX 


carried by the female-producing sperm. ‘The analysis 
is then as follows: One ‘‘dose”’ of horns (H) in the 
male produces horns, but two doses are necessary for 
the female. 

Hornless $ hX —hX 

Hormede citar ok 


F, TEXAN hornless 2 
Hh xX horned 6 








Gametes|Eggs H X —hX 


< 


of F, |Sperm H X —ha—H—h 


FP’, FEMALES F, MAuzEs 


H X H X horned H H X horned 
HX h X hornless Hh X horned 
h X H X hornless h H X horned 
hXhX _ hornless hh xX  hornless 


As pointed out by Punnett a test of the correctness 
of this interpretation is found by breeding the F, 
hornless female to a hornless male (of a hornless breed). 
It is assumed that such a female carries the factors for 
horns in a heterozygous condition; if so, then half of 
her sons should have horns, as the following analysis 
shows : 


F, Hornless 29 HX —hzx 
Hornless 6 ha—h 


h X H X hornless @ 
h X hX hornless~ 2 
hHX horned 6 
hhxX hornless 6 





THE EFFECTS OF CASTRATION 139 





Fic. 71. — Upper figure normal male guinea pig (from below), to show 
mammary glands. Lower figure, a feminized male; 7.e. castrated when 
three weeks old and pieces of ovaries transplanted beneath the skin, at Ov. 


140 HEREDITY AND SEX 


The actual result conforms to the expectation. The 
results of both of the experiments are consistent with 
the view that one factor for horns in the male produces 
horns, which we may attribute to the combined action 
of the inherited factor and a secretion from the testes 
which reénforces the action of the latter. This, how- 
ever, should be tested by castrating the 7; males. In 
the females, one factor for horns fails to produce horns, 
while two factors for horns cause their development. 

Aside from some of the domesticated animals (horses, 
cattle, dogs, cats, pigs), the only other mammals on 
which critical experiments have been made —if we 
exclude man — are the rat and the guinea pig. The 
next case is unique in that the ovary was transplanted 
to a male. 

Steinach removed the sex glands from the male 
guinea pig and rat and transplanted into the same 
animals the ovaries of the female, which established 
themselves. Their presence brought about remarkable 
effects on the castrated male. The mammary glands, 
that are in a rudimentary condition in the male, be- 
come greatly enlarged (Fig. 71). In the rat the hair 
assumes the texture of that of the female. The skele- 
ton is also more like that of the female than the male. 
The size of the feminized rats and guinea pigs is less 
than that of normal (or of: castrated) males and 
like that of the female (Fig. 72). Finally, in their 
sexual behavior, the feminized rats were more like 
females than like males. These cases are important 
because they are the only ones in which success- 
ful transplanting of the ovary into a male has been 
accomplished in vertebrates. 


THE EFFECTS OF CASTRATION 141 























Fic. 72. — Two upper figures, normal male guinea pig to left, M, and 
his brother, F, to right a feminized male. Two middle and two lower 
figures, a normal male at M, and his feminized brother, F. (After Steinach.) 





142 HEREDITY AND SEX 


OPERATIONS ON BIRDS 


In‘ striking contrast to these results with mammals 
are those with birds, where in recent years we have 
gained some definite information concerning the devel- 
opment of secondary sexual characters. 

I am fortunate in being able to refer to several 
cases — the most successful on record — carried out 
by my friend, H. D. Goodale, at the Carnegie Lab- 
oratory at Cold Spring Harbor. One ‘‘case”’ is that 
of a female Mallard duck from which the ovary was 
completely removed when she was a very young bird. 
Figure 16 illustrates the striking difference between 
the normal male and the female Mallard. In the 
spayed female the plumage is like that of the male. 

Darwin records a case in which a female duck in her 
old age assumed the characteristics of a male, and 
similar cases are recorded for pheasants and fowls. 

Goodale also removed the ovary from very young 
chicks. He found that the female developed the 
secondary sexual plumage of the cock. 

How shall we interpret these cases? It is clear that 
the female has the potentiality of producing the full 
plumage of the male, but she does not do so as long as 
the ovary is present. The ovary must therefore be 
supposed to prevent, or inhibit, the development of 
secondary sexual characters that appear therefore only 
in the male. 

The converse operation — the removal of the male 
glands from the male — is an operation that is very 
common among poultrymen. The birds grow larger 
and fatter. They are known as capons. In this case 


THE EFFECTS OF CASTRATION 143 


the male assumes his full normal plumage with all of 
his secondary male sexual characters. It is said that 
the comb and wattles and to some extent the spurs are 
less developed in the capon than in the normal male. 
But aside from this it is quite certain that the de- 
velopment of the secondary sexual plumage in the 


C0 py RI<TIE-1899 = SZ =e : x 
OY RELIABLE POULTRY SOWIE WAL Eee 
SS a 





are as = 


Fic. 73. — Male and female Seabright. Note short neck feathers and 
incomplete tail cover in male. In the Seabright cock the sickle feathers on 
back at base of tail are like those of the hen. (After “ Reliable Poultry 
Journal.’’) 


male is largely independent of the presence of the sex 
elands. : 
The method of inheritance of the secondary sexual 
characters in birds has been little studied. Daven- 
port has reported one case, but I am not sure of his in- 
terpretation.! I have begun to study the question by 
using Seabright bantams, in which the male lacks some 
1 Because it is not evident whether the secondary sexual char- 


acters as such are involved or only certain general features of 
coloration. 


144 HEREDITY AND SEX 


of the secondary sexual characters of the domestic 
races, notably the saddle feathers, as shown in Fig. 73. 

When a female Seabright was mated to a black- 
breasted game male the sons had the secondary sexual 
plumage of the father. 

In the second generation, however, some of the males 
showed imperfect development of the sickle feathers 
to various degrees—some to the extent shown by the 
Seabright. It appears that the female transmits the 
features peculiar to the male of this race. 











Seabright 2 sF — s 
Game 6 S—S 
Ss Normal 2 
F, Ss Normal 3 
Gametes | SF —sF—S—s_ Eggs 
of F, | S=—=s§ Sperm 
SSF 
SsF 
SF | Normal 9 
ssF | 
F, SS Normal 6 
Ss Normal 6 
sS Normal 6 


ss  Seabright 3 


In conclusion, then, in mammals the secondary sexual 
characters owe their development to the testes. The 
testes add something to the common inheritance. 
But in birds the ovary takes something away. 


THE EFFECTS OF CASTRATION 145 


OPERATIONS ON AMPHIBIA 


The male triton develops each year a peculiar fin or 
comb on the back and tail. Bresca has found that 
after castration the comb does not develop. If present 
at the time of castration, the comb is arrested, but 
only after several months. Certain color marks pe- 
culiar to the male are not lost after castration. If the 
comb is removed in normal males, it regenerates, but 
less perfectly in castrated males. If a piece of the 
dorsal fin of the female is transplanted to a normal male 
in normal position, it may later produce the comb under 
the influence of the testes. 

In the frog there appears at the breeding season a 
thickening of the thumb. Castrated males do not 
produce this thickening. 

If it is present in a male at the time of castration it 
is thrown off, according to Nussbaum, but according to 
Smith and Shuster its further progress only is arrested. 
According to Nussbaum and Meisenheimer injection 
of pieces of testes beneath the skin of a castrated male 
cause the thumb development to take place, or to 
continue, but Smith and Shuster question this con- 
clusion. 


Such are the remarkable relations that these experi- 
ments have brought to light. How, we may ask, do 
the sex glands produce their effect, in the one case to 
add something, in the other to suppress something ? 

It has often been suggested these glands produce 
their effects through the nervous system by means of 
the nerves to or from the reproductive organs. This 
has been disproved in several cases by cutting the 


146 HEREDITY AND SEX 


nerves and isolating the glands. The results are the 
same as when they are left intact. 

This brings us to one of the most interesting chapters 
of modern physiology, the production and influence of 
Internal Secretions. 


INTERNAL SECRETIONS 


It has become more and more probable that the effects 
in question are largely brought about by internal se- 
cretions of the reproductive organs. ‘These secretions 
are now called ‘‘hormones”’ or ‘‘exciters.’’ They are 
produced not only by glands that have ducts or outlets, 
but by many, perhaps by all, organs of the body. Some 
of these secretions have been shown to have very re- 
markable effects.. A few instances may be mentioned 
by way of example. 

The pituitary body produces a substance that has an 
important influence on growth. If the pituitary body 
becomes destroyed in man, a condition called gigan- 
tism appears. The bones, especially of the hands and 
feet and jaws, become enlarged. The disease runs a 
short course, and leads finally to a fatal issue. 

The thyroid and parathyroid bodies play an im- 
portant role in the economy of the human body 
through their internal secretions. Removal leads to 
death. A diseased condition of the glands is asso- 
ciated with at least six serious diseases, amongst them 
cretinism. 

The thymus secretion is in some way connected with 
the reproductive organs. Vincent suggests that ‘‘the 
thymus ministers to certain needs of the body before 
the reproductive organs are fully developed.” 


THE EFFECTS OF CASTRATION 147 


Extirpation of the adrenal bodies, another ductless 
gland, leads to death. Injury to these bodies causes 
Addison’s disease. 

Finally, the reproductive glands themselves produce 
internal secretions. In the case of the male mammal it 
has been shown with great probability that it is the 
supporting tissues of the glands, and not the germ-cells, 
that produce the secretion. Likewise, in the case of 
the ovary, it appears that the follicle cells of the corpus 
luteum give rise to an important internal secretion. 
If the sac-like glands are removed, the embryo fails to 
become attached to the wall of the uterus of the mother. 
If the ovary itself is removed from a young animal, 
before corpora lutea are formed, the uterus remains in 
an infantile condition. 

From a zoological point of view the recent experi- 
ments of Gudernatsch are important. He fed young 
frog tadpoles with fresh thyroid glands. ‘ They began 
very soon to change into frogs, but ceased to grow in 
size. The tadpoles might begin their metamorphosis 
in a few days after the first application of the thyroid, 
and weeks before the control animals did so.’’ 

In contrast to these effects Gudernatsch found that 
tadpoles fed on thymus grew rapidly and postponed 
metamorphosis. They might even, in fact, fail to 
change into frogs and remain permanently in the tad- 
pole condition. If thyroid extracts produce dwarfs ; 
thymus extracts make giant tadpoles that never become 
adults. 

These examples will suffice to show some of the im- 
portant effects on growth that these internal secretions 
may bring about. 


148 HEREDITY AND SEX 


OPERATIONS ON INSECTS 


The Insects constitute the third great group in which 
secondary sexual characters are common. 

The first operations on the reproductive organs were 
carried out by Oudemans on the gipsy moth, Ocnerra 
(Porthetria) dispar. The male and female are strik- 
ingly different. Oudemans removed the testes from 






CD i 


= oD \ 
j o wi A iy a 
, Nay ‘ 

Q KASy, 





---vd ff 


a, 
y TD D y 
// ey mk \, ne 
f ~ vd-- } 


| 
\ 4 ad--- / 


) | XY & 
( 
f--ad “Vs : 


f/ 
Ve 


y/ 
eSbss- 





Fia. 74. Ovaries of Lymantria (Porthetria) dispar transplanted to male. 
They have established connection with the sperm ducts. (After Kopec.) 


young caterpillars and found no change in the color, or 
size, of the male. He also removed the ovaries from 
young caterpillars, and again found no effect in the fe- 
male. The same experiments were later carried out on 
a large scale by Meisenheimer, who obtained similar 
results. Meisenheimer went further, however, and per- 
formed another operation of great interest. He removed 
the male glands from a male and implanted in their 


THE EFFECTS OF CASTRATION 149 


place the ovary of a female, while it was still in a very 
immature condition. The caterpillar underwent its 
usual growth, changed to a chrysalid, and then to a 
moth. The moth showed the characters of the male. 
The presence of the ovary had produced no effect what- 
ever on the body character of the individual. When 
this individual was dissected, Meisenheimer found that 
the ovary had completely developed. It contained 
mature eggs, and the ovary had often established con- 
nection with the outlets of the male organs that had 





Fic. 75. — Testes of Lymantria (Porthetria) dispar transplanted to female. 
They have connected with the oviducts. (After Kopec.) 


been left behind, as seen in Fig. 74, taken from Kopec’s 
description. 

The converse experiment wasalso made. The ovaries 
were removed from young caterpillars, and in their 
place were implanted the male sex glands from a young 
male caterpillar. Again no effects were produced on the 
moth, which showed the characteristic female size and 
color. On dissection the testes were also found to have 
grown to full size and to have produced spermatozoa 
play. 75). 

These remarkable results, confirmed by Kopec, show 


150 HEREDITY AND SEX 


that in these insects the essential organs of reproduc- 
tion have no influence on the secondary sexual char- 
acters of the individual. They show furthermore that 
the male generative organs will develop as well in 
the female as in the body of the male itself, and vice 
versa. 

It is evident, then, in insects (there is a similar, but 
less complete, series of experiments on the cricket), 





Fic. 76. — Papilio Memnon. 1, male; 2, 3, 4, three types of females. 
(After Meijere.) 

that the heredity of the secondary sexual characters 
can be studied quite apart from the influence of the 
sex glands. How, then, are they inherited so that they 
appear in one sex and not in the other sex? Within 
the last two or three years the inheritance of the second- 
ary sexual differences in insects has been studied. 

First, there is the case of the clover butterfly, Colias 
philodice, that Gerould has worked out, where there 


THE EFFECTS OF CASTRATION 151 


are two types of females and one kind of male 
(Fig. 66). 

Without giving the analysis of this case I may say 
that the results can be explained on a Mendelian basis. 
The peculiar feature of Gerould’s explanation is that 
two doses of the yellow-producing determiner in the 
female give yellow color — one dose gives white. In 
the male, on the other hand, one dose of yellow gives 
yellow. 

The second case is that of Papilio memnon, worked out 
by de Meijere from the experiments of Jacobson. There 
is one male type and three female types, Fig. 76. De 
Meijere accounts for the results of matings in this 
species recorded by Jacobson on the assumption of 
three factors, one for each type of female. The three 
factors are treated as allelomorphs, and therefore only 
two of them can be present in any one individual, and 
since they are allelomorphs they will pass into different 
gametes. ‘The order of dominance is Achates, Agenor, 
Laomedon. ‘The male carries these same factors, but 
they are not effective in him. Baur accounts for the 
results in a somewhat different way, but involving or- 
dinary Mendelian conceptions. 

An interesting case is that reported by Foot and 
Strobell. They crossed a female of a bug, Huschistus 
variolarvus, the male of which has a black spot on the 
end of the body (the female lacking the spot), with a 
male of Huschistus servus that lacks the spot both in 
the males and the females (Fig. 77). The daughters 
had no spot; the sons had a faint spot, less developed 
than in variolarius. When these (/) offspring were 
inbred, they obtained 249 females without a spot, 


152 HEREDITY AND SEX 


107 males with a spot (developed to different degrees), 
and 84 males without a spot. The authors give no 
explanation of their results—but they use the re- 


2 





Fic. 77. — To left, in 1, is male of Euschistus variolarius, to right male of 
E. servus. 2 and 3 show eight Ff: males; 4 shows seven F2 males from 
another mating. 


sults to discredit some of the explanations, that rest 
on the assumption that the chromosomes are the chief 
factors in Mendelian heredity. I venture, neverthe- 
less, to suggest the explanation shown on the accom- 


THE EFFECTS OF CASTRATION 153 


panying diagram (Fig. 78). The analysis rests on the 
assumption that neither one nor two doses of S in the 
female is able to produce a spot, while in the male one 
dose of S suffices. 








E. servus sX— 3X 
E.variolarius OX—S 
SX3 X = Spattess 2 | 9 SX—sX 


Fie 
$sX spr lévare SX—S 





Eogs S X—s X SXSX 
Spotl 
Sac S Xs X—S— s Sxl 
SSX spor d 
SXs xX SsX Spr ot 
Spotless ? 
sXsSxX : 
Fa. 3Xsx 


$S xX Spot & 
$sX Spot @ 
63x Spot 7 
$ 3X Spotless & 


Fic. 78. — Diagram showing a possible interpretation of the heredity 
of spot. of male when EH. servus is crossed with EH. variolarius. S=spot; 
S=no spot. 

It is very important to understand just what is meant 
by this; for otherwise it may seem only like a restate- 
ment of the facts. In the Ff, female with the formula 
SX SX, 7.e. two doses of the S factor, no spot is assumed 
to appear (nor in the hybrid female SXsX). At 
first sight it seems that a female having the formula SX 
SX is only double the male with SXs, especially if 
small s is interpreted to mean absence of spots. But 
this view, in fact, involves a misconception of what the 
factorial hypothesis is intended to mean. 


154 HEREDITY AND SEX 


To make this clearer, I have written out the case 
more fully: 


XTACB CSS Dp ASB Cae 
XCAR CSS AB: Crs iG: 


In this, as in all such Mendelian formule, the result 
(or character) that a factor produces depends on its 
relations to other things in the cell (here A BC). We 
are dealing, then, not with the relation of X to S alone, 
but this relation in turn depends on the proportion of 
both X and Sto A BC. It is clear, if this is admitted, 
that the two formule above — the one for the male 
and the other for the female—are neither identical 
nor multiples. 

It will be noted that in only one of these attempts to 
explain in insects the heredity of the secondary sexual 
characters have the factors for the characters been 
assumed to be caused by the sex chromosomes. If 
one accepts the chromosome basis for heredity, these 
results may be explained on the assumption that the 
factors lhe in other chromosomes than the sex chromo- 
somes. 

In the next case, however, that I shall bring forward 
the factors must be assumed to be in the sex 
chromosomes themselves. 

The mutant of drosophila with eosin eyes that arose 
in my cultures is the case in question. The female 
has darker eyes than the male. The experimental 
evidence shows that the factor for eosin is carried 
by the sex chromosomes. In the female it is present, 
therefore, in duplex, or, as we say, in two doses; in 
the male in one dose. 


THE EFFECTS OF CASTRATION 155 


The difference in color can be shown, in fact, to be 
due to this quantitative relation. If, for instance, an 
eosin female is mated to a white-eyed male, her 
daughters have light eyes exactly like those of the 
eosin male. The white-eyed fly lacks the eosin factor 
in his sex chromosomes (as suitable matings show), 
hence the hybrid female has but one dose of eosin, 
and in consequence her eye color becomes the same as 
the male. 

In this case a sex-linked character is also a secondary 
sexual character because it is one of the rather unusual 
cases in which a factor in two doses gives a stronger 
color than it does in one dose. 


PARASITIC CASTRATION OF CRUSTACEA 


Let us turn now to a group in which nature performs 
an interesting operation. 

Giard first discovered that when certain male crabs 
are parasitized by another crustacean, sacculina (a 
cirriped or barnacle), they develop the secondary sexual 
characters of the female. Geoffrey Smith has confirmed 
these results and carried them further in certain re- 
spects. Smith finds that the spider crab, Inachus 
mauritanicus, is frequently infected by Sacculina neglecta 
(Fig. 79). The parasite attaches itself to the crab and 
sends root-like outgrowths into its future host. These 
roots grow like a tumor, and send ramifications to all 
parts of the body of the crab. 

The chief effect of the parasite is to cause complete 
or partial atrophy of the reproductive organs of the 
crab, and also to change the secondary sexual charac- 
ters. Smith says that of 1000 crabs infected by 


156 HEREDITY AND SEX 


sacculina, 70% of both males and females showed 
alterations in their secondary sexual characters. 

As a control, 5000 individuals not infected were ex- 
amined, only one was unusual, and this one was a her- 
maphrodite (or else a crab recovered from its parasite). 





Fic. 79. — A male of Inachus maurtlanicus (upper left hand). Female of 
Inachus scorpi (lower left hand). Male of Inachuws mauritanicus carrying on 
its abdomen two specimens of Danalia curvata and a small Sacculina neglecta 
(upper right hand). Male of Inachus mauritanicus with a Sacculina neglecta 
on it (lower right hand). The abdomen and chele of the host are inter- 
mediate in character between those of an ordinary male and female. (After 
Geoffrey Smith.) 


As the figures (Fig. 80) show, the adult male has 
large claws; the female, small ones. He has a narrow 
abdomen; she has a broad one. In the male there 
is a pair of stylets on the first abdominal ring (and 
a pair of greatly reduced appendages behind them). 
The adult female has four biramous abdominal append- 
ages with hairs to carry the eggs. 


THE EFFECTS OF CASTRATION 157 





Fig. 80. — 1, adult normal male; 2, under side of abdomen of normal 


158 HEREDITY AND SEX 


The infected males ‘‘show every degree of modi- 
fication towards the female type.” The legs are small, 
the abdomen broad, the stylets reduced, and the 
typical biramous appendages with hairs appear. 

When the female crab is infected she does not change 
“toward”? the male type, although the ovary may 
be destroyed. The only external change is that the 
abdominal appendage may be reduced. 

In a hermit crab, Hupagurus meticulosus, infected by 
Peltogaster curvatus, similar results have been obtained. 
The infected male assumed the ordinary sexual char- 
acters of the female, but the females showed no change 
towards the male. 

In these cases it seems probable that the testes of 
the male suppress the development of the secondary 
sexual characters that appear ordinarily only in the 
females. The case is the reverse of that of the birds 
and different again from that of the mammals. 

In birds and mammals the secondary sexual charac- 
ters are In many cases directly dependent on the in- 
ternal secretions of the sex glands. These secretions are 
carried alike to all parts of the body, hence the absence 
of bilateral gynandromorphs in these groups. 


adult male; 3, male infected with sacculina, showing reduction of chela 
and slight broadening of abdomen; 4, 5, showing attenuated copulatory styles 
and slight hollowing out of abdomen; 6, under side of abdomen of a similar 
male specimen, showing reduction of copulatory styles and presence of 
asymmetrically placed swimmerets characteristic of female; 7, infected 
male which has assumed complete female appearance; 8, under side of 
abdomen of 7, showing reduced copulatory styles and swimmerets; 9, under 
side of abdomen of similar male specimen with well-developed copulatory 
styles and swimmerets; 10, adult female, normal; 11, under side of abdomen 
of 10, showing swimmerets and trough-shaped abdomen; 12, under side of 
abdomen of infected female, showing reduction of swimmerets; 13, immature 
female showing small flat abdomen; 14, under side of abdomen of 13, 
showing flat surface and rod-like swimmerets. (After Geoffrey Smith.) 


THE EFFECTS OF CASTRATION 159 


CONCLUSIONS 


In conclusion it is evident that the secondary sexual 
characters in four great groups, viz. mammals, birds, 
crustacea, and insects, are not on the same footing. 
Their development depends on a different relation to 
the reproductive organs in three of the groups, and is 
independent of the reproductive organs in the fourth. 
It is not likely, therefore, that their evolution can be 
explained by any one theory, even by one so broad in 
its scope as that of sexual selection. 

If, for example, in the mammals a more vigorous 
male, due to greater development of the testes, were 
‘““selected’’ by a female, the chances are that his second- 
ary sexual characters will be better developed than are 
those of less vigorous males, but he is selected, not on this 
account, but because of his vigor. Ifa male bird were 
‘“‘selected”’ on account of greater vigor, it does not 
appear that his secondary sexual characters would 
be more excessively developed than those of less vig- 
orous males, provided that his vigor were due to the 
early or greater development of the testes. If in 
birds the male by selecting the female has brought 
about the suppression of the male plumage, which is 
their common inheritance, he must have done so by 
selecting those females whose ovaries produced the 
ereatest amount of internal secretions which suppresses 
male-feathering. Moreover, he must have selected, 
not fluctuating variations, but germinal variations. 
In insects the development of the secondary sexual 
characters is not connected with the condition of the 
reproductive organs, but is determined by the complex 


160 HEREDITY AND SEX 


of factors that determines sex itself. If selection acts 
here, it must act directly on germinal variations, that 
are independent in origin of the sex-determining factor, 
but dependent on it for their expansion or suppression. 

These considerations make many of the earlier state- 
ments appear crude and unconvincing; for, they show 
that the origin of the secondary sexual characters is 
a much more complex affair than was formerly im- 
agined. 

These same considerations do not show, however, 
that if a new germinal character appeared that gave 
its possessor some advantage either by accelerating 
the opposite sex to quicker mating or by being corre- 
lated with greater vigor and thereby making more 
certain the discovery of a mate, sucha character would 
not have a better chance of perpetuation. But in 
such a case, the emphasis no longer lies on the idea 
of selection with its emotional implications, but rather 
on the appearance of a more effective machine that — 
has arisen, not because of selection, but, having arisen 
quite apart from any selective process, has found itself 
more efficient. Selection has always implied the idea 
that it creates something. Now that the evidence 
indicates that selection is not a guaranteed method 
of creating anything, its efficiency as a means of easy 
explanation is seriously impaired. 


CHAPTER VI 


GYNANDROMORPHISM, HERMAPHRODITISM, 
PARTHENOGENESIS, AND SEX 


THREE different sex conditions occur in animals and 
plants that have a direct bearing on problems of 
Heredity and Sex. 

The first condition is called Gynandromorphism — 
a condition in which one part of the body is like the 
male, and the other part like the female. 

The second condition is called Hermaphroditism — 
a condition in which the individuals of a species are all 
alike —maleness and femaleness are combined in 
the same body. ‘Two sets of reproductive organs are 
present. 

The third condition is called Parthenogenesis — 
a condition in which the eggs of an animal or plant 
develop without being fertilized. 


GYNANDROMORPHISM 


Gynandromorphs occur most frequently, in fact 
almost exclusively, in insects, where more than one 
thousand such individuals have been recorded. They 
are most abundant in butterflies, common in bees 
(Fig. 81) and ants, rarer in other groups. They 
occur relatively more often, when two varieties, or 
species, are crossed, and this fact in itself is signifi- 
cant. A few examples will bring the cases before us. 

In my cultures of fruit flies several gynandro- 

161 


162 HEREDITY AND SEX 


morphs have arisen, of which two examples are shown 
in Fig. 82. In the first case the fly is female on one 
side, as shown by the bands of her abdomen, and male 
on the other side (upper right-hand drawing). 

In the second case the fly looked like a female seen 
from above. But beneath, at the posterior end, the 
genital organs of the male are present, and normal 





Fig. 81. — A gynandromorph mutillid wasp, Psewdomethoca canadensis, 
male on right side, female on left side. 


in structure. In the latter case the fly is ostensibly 
a female, except for the male organs of reproduction. 

How can we interpret these cases? We find a 
clue, I think, in the bee. It is known that if the egg 
of the bee is fertilized, it produces a female — only 
female-producing sperms are formed. If it is un- 
fertilized, it produces a male. In the bee two polar 
bodies are produced, and after their extrusion the num- 
ber of chromosomes is reduced to half, as in ordinary 
cases. The haploid number produces a male; the 
double number produces a female. 

Boveri pointed out that if through any chance the 


GYNANDROMORPHISM 163 


entering sperm should fail to reach the egg nucleus 
before it divides, it may then fuse with one of the 
halves of the egg nucleus after that divides. From the 





Fic. 82.— Two gynandromorphs of Drosophila ampelophila. Upper 
left-hand figure, female dorsally, male ventrally (as seen in third figure, 
lower line). Upper right-hand figure, male on left side, female on right, 
and correspondingly the under side shows the same difference (lower row, 
last figure to right. Lower row from left to right; normal female, normal 
male, vertical gynandromorph and lateral gynandromorph. 


half of the egg containing the double nuclei female 
structures will develop; from the other half, contain- 
ing the half number of chromosomes, male structures 
(Fig. 83, A). Here we have a very simple explanation 
of the gynandromorphism. 


164 HEREDITY AND SEX 


There is another way in which we may imagine that 
the results are brought about. It is known that two or 


NS 


h 


Zz 


Ve 
a" 
ne 
A 


Ba) 
=e. 
Z 


NZ 
Af 
SZ 
eG 


eet 





A B 


Fig. 83. — Diagram, illustrating on left (A) Boveri’s hypothesis, on right 
(B) the author’s hypothesis, of gynandromorphism. 


more spermatozoa frequently enter the egg of the bee. 
Should only one of them unite with the egg nucleus, 
the parts that descend from this union will be female. 
If any of the outlying sperm should also develop, 


GYNANDROMORPHISM 165 


they may be supposed to produce male structures 
RUC aes 7 ) 

The first case of the fly, in which one half the body 
is male and the other female, would seem better in 
accord with Boveri’s hypothesis. In its support 
also may be urged the fact that Boveri and Herbst 
have shown that the belated sperm-nucleus may 
unite with one of the two nuclei that result from the 
first division of the egg nucleus. 

On the other hand, the second case of the fly (where 
only a small part of the body is male) may be better 
accounted for by my hypothesis. It is known that 
single sperms that enter an egg without a nucleus, 
or even with one, may divide. The two hypotheses 
are not mutually exclusive, but rather supplementary. 

Toyama has described a gynandromorph in the 
silkworm that arose in a cross between a race with a 
banded caterpillar (the female parent) and a race 
with a white caterpillar (the male parent). As shown 
in Fig. 84, the gynandromorph was striped on the left 
(maternal) side and white on the other (right) side. 
When the adult moth emerged, the left side was male 
and right side was female. Since the sperm alone 
bore the white character, which is a recessive charac- 
ter, it appears that the right side must have come 
from sperm alone. This is in accordance with my 
hypothesis. | 

In this connection, I should like to call attention to 
a relation of especial interest. Gynandromorphs are 
not uncommon in insects, rare or never present in 
birds and mammals. 

The explanation of this difference is found, I think, in 


166 HEREDITY AND SEX 


Fig. 1. Vig. It 






Fia. 84. — I, a, plain, b, striped caterpillar of silkworm. II, a, gynandro- 
morph silkworm, b, moth of same. III, wings of last. IV, dorsal view of 
same moth. V, abdomen of same. VI, end of abdomen of same moth. 
VII, normal female, and VIII, a normal male. (After Toyama.) 


HERMAPHRODITISM 167 


the relation of the secondary sexual characters to the 
‘sex glands. In insects the characters in question are not 
dependent on the presence or absence of these glands. 
Hence, when such conditions occur after fertilization, 
as those I have just considered, each part may develop 
independently of the rest. 


HERMAPHRODITISM 


In almost all of the great groups of animals a condi- 
tion is found in which complete sets of ovaries and testes 
occur in the same individual. This condition is called 
‘“‘hermaphroditism.’’ In some groups of animals, as in 
flatworms, leeches, mollusks, hermaphroditism is the 
rule, and it is the common condition in flowering 
plants. Sometimes there is only one system of outlets 
for eggs and sperm, but not infrequently each has a 
separate system. 

Here there is no problem of the production of males 
and females, for one kind of individual alone exists. 
But what determines that in one part of the body 
male organs develop, and in another part a female 
system ? 

Two views suggest themselves, either somatic segre- 
gation, or regional differentiation. By somatic seg- 
regation I mean that at some time in the development 
of the embryo — at some critical division — a separa- 
tion of chromosomes takes place so that an egg-produc- 
ing group and a sperm-producing group is formed. 
There is no direct evidence in support of this view. 

Another view is that the formation of ovary and 
testis is brought about in the same way as _ all 
differentiations of body organs, as for example the 


168 HEREDITY AND SEX 


formation of liver and lungs and pancreas from the 
digestive tract. The following case may perhaps 
be considered as supporting such an hypothesis. In 
a hermaphroditic worm, Criodrilus lacuum the ovaries 
lie in the thirteenth and the testes in the tenth and 
eleventh segments. If the anterior end be cut off, a 
new one regenerates, as shown by Janda (Fig. 85), 








Fia. 85. 
system; 2-5, regenerated anterior ends. (After Janda.) 


1, anterior end of normal criodrilus, showing reproductive 


in which the ovaries and testes reappear approximately 
in their appropriate regions. It is true their location 
is more liable to vary than in the normal worm, but 
this is unimportant. The important point is that 
they must be produced from parts of the body that 
have never produced them before, and it is unlikely 
therefore that any preparation for this casualty would 
have been made. The location and differentiation 


HERMAPHRODITISM 169 


of these organs may seem to depend on the same 
‘“‘relation-of-the-parts-to-each-other ’? on which - all 
somatic differentiation depends. 

If this were the correct interpretation then the prob- 
lem of sex in hermaphrodites would appear in a different 
light from the problem of sex in species in which males 
and females occur, and the appeal would be made to an 
entirely different principle. 

In cases where a sexual generation alternates with 
a hermaphroditic generation, the problem of the two 





Fia. 86. — Rhabditis nigrivenosa, male (left) and female (right). (After 
Leunis.) 


sexes reappears. There is but one case in animals 
that has been adequately worked out. A nematode 
worm, Rhabditis nigrovenosa, lives as a parasite in 
the lungs of frogs. It is an hermaphrodite. Its 
eggs give rise to another generation that lives in mud 
and slime. In this generation two kinds of individuals 
are present — true males and females (Fig. 86). The 
females produce eggs, that are fertilized, and develop 


170 HEREDITY AND SEX 


into the hermaphrodites which find their way again 
into the lungs of frogs. 

Boveri and Schleip have worked out the history 
of the chromosomes in this case. The cells of the 





Fic. 87. — Chromosomes of Angiostomum. (A), odgonia; (B), equa- 
torial plate of first maturation division; (C), young spermatocyte; 
(D), first spermatocyte division in metaphase; (#), same in anaphase; 
(Ff), spermatocyte of second division; (G@), and (#), division of same; 
(1), and (K), loss of X at plane of division; (ZL), first segmentation division 
of a male embryo; two sets of chromosomes (5 and 6 = 11 respectively) 
separate; (JM) equatorial plate of dividing cell of female embryo = 12 
chromosomes; (N), same from male embryo = 11 chromosomes. (After 
Schleip.) 


hermaphrodite have twelve chromosomes (Fig. 87). 
The eggs, after extruding two polar bodies, have 
six chromosomes. The spermatozoa that develop 
in the body of the same animal have six or five chro- 
mosomes each, because one chromosome is lost in half 


HERMAPHRODITISM 171 


of the cells by being left at the dividing line between 
the two cells. We can understand how two kinds 
of individuals are produced by the hermaphrodites 
from the two classes of sperm combining at random 
with the eggs. 

These two kinds of individuals are females with 
twelve chromosomes, and males with eleven chromo- 
somes. How then can we get back to the hermaph- 
roditic generation? Boveri and Schleip suggest that 
the males again produce two kinds of spermatozoa, — 
they have shown this to be the case in fact, — and that 
the male-producing spermatozoa become function- 
less. Here we have at least an outline of some of 
the events in the life cycle of this worm in relation 
to the chromosomes, but no explanation of hermaph- 
roditism. , 

Turning to plants, there are the interesting experi- 
ments of the Marchals with mosses. They show that 
a hermaphroditic or sporophyte plant has the factors 
for maleness and femaleness combined as a result 
of fertilization; while in the formation of the spores 
the factors in question are separated. 

Blakeslee has found somewhat similar relations in 
certain of the molds. The spores in molds contain 
more than one nucleus, therefore it is not clear how 
segregation in the sense used for other cases applies 
here. 

In the flowering plants that are hermaphroditic 
we have Correns’ experiments, in which he crossed an 
hermaphroditic type of Bryonia alba with a type 
B. dioica in which the sexes are separate. The 
cross when made one way gives only females, while 


172 HEREDITY AND SEX 


the reciprocal cross gives males and females in equal 
numbers. Correns’ interpretation is shown in the 
lower part of the next diagram. 


Bryonia dioica and B. alba 


B. dioica 9 by B. alba ¢ B. alba g by B. dioica $ 


Females Females and Males 





Correns’ Explanation 








F—F B. dioica 9 (FM)—(FM) B. alba 9 
(FM)—(FM) B. alba 8 F—M _  B. dioica ¢ 
F(FM) female F( FM) female 


M(FM) male 


It is based in the first case on the assumption that 
the hermaphroditic condition of B. alba is recessive to 
the dicecious condition of B. dioica, and that the female 


—— Lyehnis dioica 
Female FF Male FF. Hermaph. FH. 


F—F femate F — HH herm.ovue 
F—H _  henn.pollen F—H_ « pollen 
FF females FF females 
FH herm. FH herm. 
F—H herm. ovale 
F— male pollen 
FF female 
Ff male 


Fia. 88. — Diagram to illustrate G. H. Shull’s results on Lychnis dioica. 
The symbols here used are not those used by Shull. Two types are assumed 
not to appear, viz. HH and Hf. 


PARTHENOGENESIS 173 


dioica is homozygous for the sex factor. The recip- 
rocal cross is explained on the basis that maleness 
dominates femaleness. It is difficult to bring this view 
into line with other hypotheses of sex determination. 

Shull obtained as a mutant a hermaphroditic plant 
of Lychnis dioica. The next diagram (Fig. 88) gives 
the principal facts of his crosses. When a female 
plant is fertilized by the pollen of the hermaphrodite, 
two kinds of offspring are produced — females and 
hermaphrodites. When the hermaphrodite is_ self- 
fertilized, the same two classes are produced. When 
the ovule of the hermaphrodite is fertilized by the 
pollen from the male plant, two kinds of offspring 
are again produced — female and male. Shull’s inter- 
pretation is too involved to give here. In the diagram 
the scheme is worked out on the purely arbitrary 
scheme that the hermaphrodite is YH, in which F 
is a female factor, and H a modification of it which 
gives hermaphroditism. This leads to the further 
assumption that ovule and pollen, bearing the H 
factor, cannot produce a plant nor can the combination 
fH. This scheme is only intended as a shorthand way 
of indicating the results, and not as an interpretation 
of actual conditions. 


PARTHENOGENESIS 


A third important condition in which the heredity 
of sex is involved is found in parthenogenesis. 

It has long been known to biologists, that in many 
different species of animals and plants eggs develop 
without being fertilized. This is recognized as a 
regular method of propagation in some species. The 


174 HEREDITY AND SEX 


eggs are produced in the same way as are other eggs. 
They are produced in ovaries that have the same 
structure as the ovaries that give rise to ordinary 
eggs. Parthenogenetic eggs differ from spores, not 
only in their origin in an ovary, but in that they also 
produce polar bodies like ordinary eggs. Most, but 
not all, parthenogenetic eggs give rise, however, to 
only one polar body. Some of them at least fail to 
pass through the stage of synapsis, and, in consequence, 
they retain the full number of chromosomes. 





Fig. 89. — Miastor, sexual male and female (to right). Three larve 
with young inside (to left). 


A few examples will bring the main facts before us. 

A fly, miastor, appears in the spring of the year 
under two forms, male and female (Fig. 89). The eggs 
are fertilized and each produces a worm-like larva. 
This larva produces eggs while still in the larval stage. 
The eggs develop without fertilization, and produce 
new larve, which repeat the process. This method 
of propagation goes on throughout the rest of the 
year until finally the adult winged flies reappear. 

The bee is the most remarkable instance, for here 


PARTHENOGENESIS 175 


the same egg will produce, if it is fertilized, a female 
(queen or worker), or, if it is not fertilized, a male 
(drone). If the queen deposits an egg in a cell of the 
comb that has been built for a queen or a worker, she 
fertilizes the egg ; if in a drone cell, the egg is not fertil- 
ized. We need not conclude that the queen knows 
what she is about — the difference in shape of the drone 
cell may suppress the reflex, that in the other cases 
sets free the sperm. 

The case of the bee has attracted so much attention 
that I may be allowed to pause for a moment to point 
out some of the most recent results connected with the 
formation of the germ-cells. 

The egg produces two polar bodies — the process 
being completed after the sperm has entered the fer- 
tilized egg (Fig. 90). Eight chromosomes are present 
at each division. Eight remain in the egg (these are 
double chromosomes — therefore 16). The sperm 
brings in 8 (double) chromosomes so that the female 
comes to have 16 single chromosomes in her cells. There 
is only one kind of spermatozo6n, as shown by the figure, . 
for the first spermatocyte division is abortive — all 
the chromosomes passing into one cell only, and the 
second division gives rise to a small cell, that does not 
produce a spermatozoon, and a large cell that becomes 
a spermatozoon. 

If the egg is not fertilized, it also gives off two polar 
bodies. It has 8 chromosomes left. The male de- 
velops with the half number. The formula for the 
female will be XABCD XABCD and for the male 
XABCD. 

If the bee conforms to the ordinary type for insects, 





176 HEREDITY AND SEX 


we may suppose that one sex chromosome is present 
in the male or at least one differential factor for sex, 
and that it is present in all the functional spermato- 
zoa. The female will then have two such chromo- 
somes and come under the general scheme for insects. 


‘aan Le - & 
1“) . “ff i 
on fe 
%eae tee nes Mo ® 
je ° 
ee? @ g 


(ly 
8 


I | 
ey 





r e 3 74 
310 16 +16 = 329 


Fig. 90. — Odgenesis and spermatogenesis in bee. Four upper figures, 
A-—D, show formation of first (4), and second (B) polar bodies. Only inner 
group of chromosomes remains (C) to form egg nucleus. Entrance of sperm 
nucleusin D. E shows scheme of these two divisions involving eight double 
(82) chromosomes. F, first and second spermatocyte divisions, the first, 
a, b, abortive, leading to pinching off of a small cell without a nucleus, the 
second, c, c, leading to formation of a large (functional) and an abortive 
cell (above). 


In the gall fly, Neuroterus lenticularis, partheno- 
genetic females appear early in the spring. Their eggs 
produce females and males — the second generation. 
The fertilized eggs of these females give rise the follow- 
ing year to the spring parthenogenetic females. Don- 
caster has found that each parthenogenetic female 


PARTHENOGENESIS 177 


produces eggs, all of which give rise to females or else 
to males. In connection with this fact he finds that 
the eggs of some females do not give off any polar 
bodies but retain the full number (20) of chromosomes. 





rhe 
2% 20 





f x fa oe 
Mm & 
a) 








at 
| ‘ ee A 
Kot fre Sy a 
* Rae Fi 
ase oie \ ‘s Nw \ Sperm 
10 pee + NBs, 4 vw / 
a" /- 40 . 
Cc D 
Fia. 91. — Illustrating chromosome cycle in Neuroterus. <A, one type of 


spring female, whose eggs (containing 20 chromosomes) produce no polar 
bodies. Only sexual females result. B, the other type of spring female 
whose eggs form two polar bodies, leaving 10 chromosomes in egg. These 
eggs giverise to males. C, ripening of egg of sexual female (2d generation), 
and D, spermatogenesis of male (second generation). 


These eggs produce sexual females (in left-hand side 
of Fig.91). From the eggs of other parthenogenetic fe- 
males two polar bodies are given off, and the half (10) 
number of chromosomes is left in the egg (see right-hand 
side of Fig. 91). These eggs produce males. ‘The life 


178 HEREDITY AND SEX 


cycle finds its explanation in these relations except that 
the origin of the two kinds of parthenogenetic females 
is unexplained. If we were justified in assuming that 
two classes of female-producing sperm are made in the 
male, even this point would be cleared up, for in this 


* Phill CHOU COIYUECOMMS 






Storm SHether 





A Mig veered. \p reclt (C64. 


Fia. 92. — Life cycle of Phylloxera caryecaulis. 


way the two classes of parthenogenetic females could 
be explained. 

In another group of insects, the aphids and phyllox- 
erans, the situation is different. 

In the phylloxerans of the hickories there emerges 
in the spring, from a fertilized egg, a female known as 
the stem mother (Fig. 92). She pierces a young leaf 


PARTHENOGENESIS 179 


with her proboscis, which causes a proliferation of the 
cells of the leaf. Eventually the leaf cells grow so fast 
that the stem mother is overarched in the gall that she 
has called forth. 

Inside the gall she begins to lay her eggs. From these 
eggs emerge young individuals that remain in the gall 
until they pass their last molt, when they become winged 
migrants. Externally all the migrants are alike; but 
if they are dissected, it will be found that some of them 
have large eggs, some small eggs. But all the offspring 
of the same mother are of one or of the other sort. 

The migrants crawl out of the opening in the gall and 
fly away. Alighting on other hickories, they quickly 
deposit their eggs. From the large eggs the sexual 
females emerge. ‘They never grow any bigger than the 
ege from which they hatched. In fact, they have no 
means of feeding, and contain only one large egg with 
a thick coat —an egg almost as large as the female 
herself. 

From the small eggs of the migrants, minute males 
are produced —ripe at their birth. They fertilize 
the sexual female. She then deposits her single egg on 
the bark of the hickory tree. From this egg (that lies 
dormant throughout the entire summer and following 
winter) there emerges next spring a female, the stem 
mother of a new line. 

Here we find three generations in the cycle — two 
of which reproduce by parthenogenesis. The first 
parthenogenetic generation gives rise to two kinds of 
individuals — one makes large eggs, the other small 
eggs. The large eggs produce sexual females, the small 
eggs males. 


180 HEREDITY AND SEX 


A study of the chromosomes has explained how some 
of these changes in successive generations are brought 
about. It has explained, for instance, how males are 
produced by parthenogenesis, and why the sexual egg 
produces only females. Let us take up the last point 
first. 

When the spermatocytes are produced, we find, as in 
many other insects, that at one division a sex chromo- 
some passes to one cell only (Fig. 93). Two classes of 
cells are produced —one with three, one with two, 
chromosomes. The latter degenerates, and in conse- 
quence only the female-producing spermatozoa become 
functional. All fertilized eggs give rise therefore to 
females. 

The second point that has been made out concerns 
the production of the male. When the small egg 
produces its single polar body, all of the chromosomes 
divide, except one, which passes out entire into the 
polar body. In consequence the number of chromo- 
somes left in the egg is one less than the total number. 
In a word, there are five chromosomes in the male, 
while there are six chromosomes in the female (Fig. 93). 
By throwing out one chromosome, the change is effected. 
The chromosome is the mate of the sex chromosome, 
that appeared as a lagging chromosome in the spermato- 
genesis. 

In the large egg no such diminution takes place, 
consequently the diploid number of chromosomes is 
present in the female. These unite in pairs and are 
reduced to three when the two polar bodies of the 
sexual egg are produced. 

We see that by means of the chromosomes we can 


PARTHENOGENESIS 181 


bring this case into line with the rest of our informa- 
tion bearing on the relation of the chromosomes to sex. 
One important point still remains to be explained. 
What causes some of the migrants to produce large 


PHYLLOXERA CARYECAULIS 
Polar Plate @@ © 
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a 
Letarv Plate 
O a7 


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v v 
at eR ce en Ss eC Nell ae 
oe eo Cree. 
2 oa| ft 10 Filan, Spiimle 
7 ge © Og? Oe Sy 
| y 
Bae “aN feat 


Sow 
Sats 
SS 
SAS 
& 
O 

; 


Fia. 93. — Chromosomal cycle of P. caryecaulis. 


182 HEREDITY AND SEX 


eggs and others small eggs? There must be, in all prob- 
ability, two kinds of parthenogenetic eggs produced 
by the stem mother—or at least there must be two 
kinds after the single polar body has been extruded.' 

In another group of animals, the daphnians, parthen- 
ogenetic species occur, that, in certains respects, are 
like the phylloxerans ; but these species illustrate also 
another relation of general interest. 

The fertilized winter egg produces always a female, 
the stem mother, which gives rise by parthenogenesis 
to offspring like herself, and the process may continue 
a long time. Each female produces one brood, then 
another and another. The last broods fail to develop, 
and this is a sign that the female has nearly reached 
the end of her life. 

But a parthenogenetic female may produce one or two 
large resting eggs instead of parthenogenetic females, 
and the same female may at another time produce a 
brood of males. The large resting eggs are inclosed 
in a thick outer protecting case. They must be fer- 
tilized in order to develop, yet they do not develop at 
once, but pass through an enforced, or a resting, stage 
that may be shortened, if the egg is dried and then 
returned to water. 


1'The explanation may be found in the occurrence of two types 
of males — one type with two sex chromosomes, the other with one — 
two such types were actually figured in my paper. From the type 
with two sex chromosomes a stem mother would be produced with 
four sex chromosomes (two coming from the sexual egg). 
She would give rise to migrants with large eggs. From the type 
with one sex chromosome a stem mother would arise that produced 
small eggs with three sexchromosomes. According to whether two 
or one went out into the polar bodies of the small eggs, the two types 
of male would be reproduced. 


PARTHENOGENESIS 183 
















































































@ Veer QD Untisted female  @ Wale 
Fic. 94. — Life cycle of Simocephalus; successive broods in horizontal 


lines, successive generations in vertical lines. (After Papanicolau.) 


184 HEREDITY AND SEX 


In this life history we do not know what changes 
take place in the chromosomes. It has, however, often 
been claimed in this case that the transition from par- 
thenogenesis to sexual reproduction is due to changes in 
the environment. 

In fact, this is one of the stock cases cited in the older 
literature to show that sex is determined by external 
agents. It was said, that if the environment causes 
males to appear, then sex is determined by the environ- 
ment. But as a matter of fact, in so far as changes in 
the environment affect this animal, they cause it to 
cease reproducing by parthenogenesis, and induce sexual 
reproduction instead. The evidence is consistent in 
showing that any external change that affects the 
mode of reproduction at all calls forth either sexual 
eggs or males. The machinery of parthenogenesis 
is switched off, and that for sexual reproduction is 
turned on. 

The discrepancies that appear in the older accounts 
are probably due, as Papanicolau has shown, to dif- 
ferent observers using females that belong to different 
phases of the parthenogenetic cycle. Papanicolau, 
starting in each case with a winter egg, finds that as 
successive broods are produced the color of the par- 
thenogenetic eggs can be seen to undergo a progressive 
change from blue to violet. As the change progresses 
the chance that males and sexual eggs (‘‘females’’) will 
appear is greater. Until towards the end of the life of 
the individual the males and females come, as it were, 
of themselves (Fig. 94). If, however, individuals of 
successive broods are subjected to cold, it is found that 
while earlier broods do not respond, later ones respond 


PARTHENOGENESIS 185 


more and more easily and change over to the sexual 
phase of the cycle. 

What has just been said about the successive broods 
might be said equally of the first-born offspring of the 
successive generations, as Papanicolau’s table shows 
(Fig. 94). Later born offspring respond more readily 
than do those that are historically nearer to the fer- 
tilized egg. 

It seems to me that these results become a little less 
obscure if we suppose some substance is produced during 
fertilization, that is carried by successive broods and 
successive descendants in an ever decreasing amount. 
As it becomes used up, the change is indicated by the 
color change in the egg. When it disappears, the sexual 
phase comes on. Its disappearance may be hastened 
by cold or by starvation. 

A third type, Hydatina senta (Fig. 95), an almost 
microscopic worm-like animal belonging to the rotifers, 
reproduces by parthenogenesis. 

The resting egg always gives rise to a parthenogenetic 
female, which also reproduces by parthenogenesis. 
Whitney has obtained 500 generations produced in this 
way. But from time to time another kind of individual 
appears. She is externally like the parthenogenetic 
female, but has entirely different capacities. Her 
eggs may be fertilized, and if they are they become 
resting eggs inclosed in a hard case. ‘The sperm enters 
when the eggs are immature and still in the ovary of 
the mother. The presence of a spermatozo6n in an egg 
determines that the egg goes on to enlarge and to pro- 
duce its thick coat. But if perchance no males are 
there to fertilize the eggs, this same female produces a 


186 HEREDITY AND SEX 


crop of male eggs that develop into males without 
being fertilized at all. 
There are several facts of unusual interest in the 


HYDATINA SENTA 






Fevrlthen CGCHIALE 
(4) 


ay, coal ) 
Pa R 


Thale - egg pre Lcer. 


Fia. 95. — Life cycle of Hydatina senta. 


life history of hydatina, but we have occasion to consider 
only one of them. It has been claimed in this case 
also that external conditions determine the production 
of males, A more striking example of the erroneous- 


PARTHENOGENESIS 187 


ness of this general conclusion would be hard to find ; 
for, in the first place, as we have seen, the same indi- 
vidual that produces males will produce out of the same 
eges females if she happens to be fertilized. In 
the second place the older evidence which was supposed 
to establish the view that certain specified changes in 
the environment cause the production of males has 
been overthrown. 

The French zodlogist, Maupas, is deserving of high 
praise for working out some of the most essential facts 
in the life cycle of hydatina, and for opening up a 
new field of investigation. But the evidence which 
he brought forward to show that by a low tempera- 
ture a high production of males is caused has not 
been confirmed by very careful and extensive repeti- 
tion of his experiments by Whitney and by A. F. 
Shull. The evidence that Nussbaum obtained which 
seemed to him to show that food conditions de- 
termined the production of males has likewise not 
borne the test of more recent work by Punnett, Shull, 
and Whitney. 

It has been found, however, that the production of 
the sexual phase of the cycle can be suppressed so that 
the animals continue almost indefinitely propagating 
by parthenogenesis. In several ways this may be 
accomplished. If hydatina is kept in a concentrated 
solution of the food culture, the sexual phase does not 
appear. The result has nothing to do with the abun- 
dance of food, for, if the food be filtered out from 
the fluid medium, the filtrate gives the same result. 
The following table given by Shull shows this very 
clearly. 


188 HEREDITY AND SEX 








OLD CULTURE FILTRATE 





SprRiING WATER 
One-fourth One-half Three-fourths Undiluted 





99139 | 999 | a eeiaame 





| | 
350 | 8 | 362 | 0 | 337 











%of oe! 12.8 5.7 4.1 2.1 0.0 

















Showing the number of male- and female-producers in the progeny 
of five sister individuals of Hydatina senta, one line being reared in 
spring water, the others in various concentrations of the filtrate 
from old food cultures. 


The extent of dilution of the medium is seen to be 
directly in proportion to the number of sexual forms 
that appear. If the solution be dried and the dry 
substance added to ordinary water, the same end is 
attained. 

It has not been possible to reverse the process and 
produce more sexual forms than are produced under 
ordinary conditions. This seems to mean that achange 
may be effected in one direction and not in the other. 
We cannot make a locomotive go faster than its mech- 
anism permits, with the most favorable conditions of 
fuel, oil, roadbed, and engineer; but if we put in stones 
in place of coal, we can bring it to a standstill. 


ARTIFICIAL PARTHENOGENESIS 


We have now considered some of the most striking 
examples of natural parthenogenesis in the animal 
kingdom. The facts show that fertilization of the egg 
is not im itself essential for development. The in- 


ARTIFICIAL PARTHENOGENESIS 189 


dividuals that develop from parthenogenetic eggs are as 
vigorous as those from eggs that have been fertilized. 
We have seen that such eggs without being fertilized 
are capable of producing sexual females and males. 
In one case, at least, we have seen how the process is 
accomplished. 

When we review the facts of natural parthenogenesis, 
we find certain relations that arrest our attention. 

Most parthenogenetic eggs give off only a single 
polar body, while fertilized eggs without exception give 
off two polar bodies’ This difference is clearly con- 
nected with the fact that in parthenogenetic eggs the 
full number or diploid number of chromosomes is re- 
tained by the egg.! In fertilized eggs half the chromo- 
somes are thrown out in one of the two polar bodies. 
The number is made good by the chromosomes brought 
in by the spermatozoon. 

But this difference does not in the least explain nat- 
ural parthenogenesis; for we have experimental evi- 
dence to show, that an egg will develop when only half 
the number of chromosomes is present — one set will 
suffice. 

There is another fact about parthenogenetic eggs 
that has, I believe, been generally overlooked. Many 
of these eggs begin to develop into an embryo before 
they reach the full size of the fertilized eggs of the 
same species. This is true at least of the eggs of aphids, 
phylloxerans, daphnians, and rotifers. I interpret this 

1 According to my observations on aphids and phylloxerans, the 
synapsis stage is omitted in parthenogenetic eggs, hence there is 
no union (or reduction) of the chromosomes. The omission of this 


stage may have something to do with parthenogenesis, although it 
is not evident what the relation may be. 


190 HEREDITY AND SEX 


to mean that the eggs begin their development be- 
fore there has been produced over their surface a 
layer that in the mature egg seems to have an im- 
portant influence in restraining sexual eggs from de- 
velopment. | 

This brings us at once to a consideration of what 
keeps sexual eggs from developing until they are fer- 
tilized. 

In recent years a great variety of methods has been 
discovered by means of which sexual eggs can be made 
to develop without fertilization. This process is 
called artificial parthenogenesis. We owe especially 
to Professor Jacques Loeb the most successful accom- 
plishment of this important feat. The discovery in 
his hands has led to very great advances in our 
understanding of the developmental process. 

The chief importance of Loeb’s work les, in my 
opinion, not only in the production of embryos with- 
out fertilization (nature has long been conversant 
with such methods), but in other directions as well. 

First, it has thrown light on the nature of the in- 
hibitory process that holds back the sexual egg from 
developing until the sperm enters. 

Second, the information gained in this way tells us 
something of how the sperm itself may act on the egg 
and start it on its course. 

Third, it opens up the opportunity of studying cer- 
tain problems connected with the determination of sex 
that can be gained in no other way. 

Let me attempt briefly to elaborate some of these 
points. 

In many eggs, perhaps in all, a membrane is produced 


i. . 
. ae 
r ha 


ARTIFICIAL PARTHENOGENESIS 191 


at the surface of the egg immediately after the 
sperm has entered. Here we have ocular evidence 
that fertilization effects a change in the surface layer 
of the egg. 

It has been shown that after this membrane is formed, 
the permeability of the egg to salts and other agents is 
affected and that the processes of oxidation are greatly 
accelerated. | 

In other words, the interior of the unfertilized egg is 
separated by means of its membrane from many things 
‘in the surrounding medium — oxygen and the salts in 
sea-water, for example. The egg after fertilization 
lives in a new world. 

These same changes are brought about by those 
external agents that cause artificial parthenogenesis. 
But what an array of substances can cause the effect ! 
Many kinds of salts and of drugs, acids and alkalis, 
heat or cold, shaking or even sticking the surface of 
the egg with a minute needle. 

Loeb has shown that development depends not 
only on a change in the surface of the egg, but on other 
changes also. Hence his most successful methods are 
those in which two agents are applied successively 
to the egg — one affects primarily the surface, the other 
the interior of the egg. If, for example, the eggs are 
placed in a solution of a fatty acid, the membrane is 
produced. The egg is then removed to pure sea 
water from which oxygen has been driven out and left 
there for three hours. After its return to sea water it 
will produce a normal embryo. 

If, instead of putting the egg into water without 
oxygen, a hypertonic solution of salts is used (50 ce. 


192 HEREDITY AND SEX 


of sea water plus 8 cc. of 2144 NaCl), the development 
may be carried through. 

Loeb concludes that the oxidations set up in the egg 
by a change in its outer surface affect the egg itself 
injuriously; and unless they are removed or the 
effects are counterbalanced by some other change 
(as when a hypertonic solution is used) the egg goes 
to pieces. Hence he believes that the sperm has a 
double réle in fertilization. First it changes the surface 
layer and increases in consequence the oxidations 
in the egg; second, the sperm brings into the egg some 
substance that counteracts poison produced by the 
oxidation itself. 

This is what fertilization accomplishes from a 
phystological point of view. In addition, we have 
seen that fertilization brings into the egg certain ma- 
terials whose presence affects the characters of the 
individuals that develop from it. This is what fertili- 
zation does from the point of view of the student of 
heredity. 

Let us turn for a moment, in conclusion, to the 
question of sex of anvmals that come from artificially 
parthenogenetic eggs. 

In natural parthenogenesis such eggs may de- 
velop into males, sexual females, or parthenogenetic 
females. 

But in artificial parthenogenesis the egg has already 
undergone reduction in its chromosomes and is repre- 
sented by half of the female formula as far as the 
chromosomes are concerned. The half formula will 
be XABC for the type with homozygous female. 
Since the egg has one X it may be expected to become 


ARTIFICIAL PARTHENOGENESIS 193 


a male, but if sex is a relation of X to ABC, one cannot . 
be certain that it might not be a female. 

In cases where the female is heterozygous for the 
sex factor, as in birds and some sea urchins, the formula 
for the female would be XABCD — YABCD and for 
the male YABCD— YABCD. There would be two 
types of eggs, XABCD and YABCD. The former 
might be expected to produce a female, the latter prob- 
ably a male if such eggs were incited artificially to 
develop. 

Concerning the sex of the embryos so far produced 
by artificial parthenogenesis, we know of only two 
cases. These two cases are Delages’ result for the 
sea urchin, in which he got one male, and Loeb’s and 
Bancroft’s case for the frog, in which they believe that 
the two young obtained were females. 

What to expect on theoretical grounds is uncertain. 
We have only two facts that bear on the question. 
In the parthenogenetic eggs of the aphid, with the for- 
mula XABC ABC we get amale. In the case of the 
bee the formula is X ABC, which also givesa male. All 
else is hypothetical and premature, but if these two 
formule are correct, it appears that one X gives a 
male and that maleness is not due to a quantitative 
relation between X and one or two sets of the other 
chromosomes. It is the quantity of something in X, 
not the relation of this to the rest of the chromosomes. 


CHAPTER VII 
FERTILITY 


Darwin’s splendid work on cross- and self-fertiliza- 
tion, his study of the mechanism of cross-fertilization in 
orchids, and his work on the different forms of flowers 
of plants of the same species, mark the beginning of 
the modern study of the problem of fertility and 
sterility. Darwin carried out studies on -the effects 
of cross-fertilization in comparison with self-fertilization 
and reached the conclusion that the offspring resulting 
from cross-fertilization are more vigorous than the 
offspring from self-fertilization. No one can read his 
books dealing with these questions without being 
impressed by the keenness of his analysis and the 
open-minded and candid spirit with which the prob- 
lems were handled. Since Darwin’s time we lave not 
advanced very far beyond the stage to which Darwin 
carried these questions. We have more extensive 
experiments and some more definite ways of stating 
the results, but Darwin’s work still stands as the most 
important contribution that has been made to this 
subject. 

The credit of the second advance belongs to Weis- 
mann. His speculations concerning the effects of 
mixing of the germ-plasms of the two individuals, 
that combine at the time of fertilization, not only 
aroused renewed interest in the nature of the process 
of sexual reproduction, but brought to light also the 

194 


FERTILITY 195 


effects of recombination of the different sorts of qualities 
contained in the parental strains. His attack on the 
hypothesis of rejuvenation that was so generally held 
at that time did very great service in exposing the 
mystical nature of such an imagined effect of cross- 
fertilization. In particular, Weismann’s endeavor to 
connect the theory of recombination with the facts 
of maturation of the egg and sperm has opened our 
eyes to possibilities that had never been realized before. 
His work has led directly to the third advance that 
has been made in very recent years, when the results 
of Mendelian segregation have been applied directly 
to the study of fertility and sterility. 

As I have said, Darwin’s work showed that cross- 
fertilization is generally beneficial. The converse 
proposition has long been held that continued inbreed- 
ing leads to degeneration and to sterility. This opinion 
rests largely on the statements of breeders of domesti- 
cated animals and plants, but there is also a small 
amount of accurate data that seems to support this 
view. I propose first to examine this question, and 
then consider what cross-fertilization is supposed to do, 
in the light of the most recent work. 

Weismann inbred white mice for 29 generations, 
and Ritzema-Bos bred rats for 30 generations. In 
each case the number of young per litter decreased 
in successive generations, more individuals were sterile 
and many individuals became weakened. This evi- 
dence falls in line with the general opinion of breeders. 

On the other hand, we have Castle’s evidence on 
inbreeding the fruit fly through 59 generations. He 
found some evidence of the occurrence of sterile pairs 


196 HEREDITY AND SEX 


(mainly females), but we must be careful to distinguish 
between the appearance of sterile individuals in these 
cultures and the lessened fertility that may be shown 
by the stock in general. The recent work of Hyde on 
these same flies has shown that the appearance of 
sterile individuals may be an entirely different question 
from that of a decrease in general fertility. The 
latter again may be due to a number of quite different 
conditions. Castle and his co-workers found that the 
sterile individuals could be eliminated if in each genera- 
tion the offspring were selected from pairs that had not 
produced sterile individuals. Hyde has found, in 
fact, that one kind at least of sterile females owe their 
sterility to a definitely inherited factor that can be 
eliminated as can any other Mendelian recessive | 
trait. Moenkhaus, who has also extensively studied the 
problem of inbreeding in these flies has likewise found 
that his strains could be maintained at their normal 
rate of propagation by selecting from the more fertile 
pairs. 

If we eliminate from the discussion the occurrence 
of sterile individuals, the question still remains whether 
the output of the fertile pairs decreases if inbreeding 
is carried on through successive generations. There 
is some substantial evidence to show that this really 
takes place, as the following figures taken from Hyde’s 
results show. 


Py By Fs Ry. Bs Oi 
368 209 “191° 184 - 65 119 156 


At the end of thirteen generations the fertility 
of the stock was reduced by half, as determined in this 


FERTILITY 197 


ease by the average number of flies per pair that 
hatch. But this is not a measure of the number of 
eggs laid or of those that are fertilized. , 

Whether inbreeding where separate sexes exist 1s sim- 
ilar to self-fertilization in hermaphroditic forms is not 
known. Darwin gives results of self-fertilization in [po- 
mea purpurea for ten generations. The effects vary so 
much in successive generations that it is not possible 
to state whether or not the plant has become less 
fertile. His evidence shows, however, that the cross- 
fertilized plants in each of the same ten generations 
are more vigorous than the self-fertilized plants, but 
this does not prove that the latter deteriorated. 

The problem has been studied in other ways. Some 
animals and plants propagate extensively by partheno- 
genesis; others by means of simple division. 

Whitney and A. F. Shull kept parthenogenetic strains 
of Hydatina senta for many generations. Whitney 
carried a strain of this sort through 500 generations. 
Towards the end the individuals became weak, the 
reproductive power was greatly diminished, and finally 
the strain died out. No attempt was made to breed 
from the more fertile individuals, although to some 
extent this probably occurred at times. If we admit 
that weakened individuals: appear sometimes in these 
lines and their weakness is inherited, then each time 
such an individual happened to be picked out a step 
downward would be taken; when the more fertile 
individuals chanced to be selected, the strain would be 
temporarily held at that level. But on the whole 
the process would be downwards if such downward 
changes are more likely to occur than upward ones. 


198 HEREDITY AND SEX 


This is an assumption, but perhaps not an unreasonable 
one. Let me illustrate why I think it is not unreason- 
able. If the highest possible point of productivity 
is a complex condition due to a large number of things 
‘that must be present, then any change is more likely 
to be downward, since at the beginning the high-water 
mark had been reached. In time casual selection would 
be likely to pick out a poor combination —if this hap- 
pened once the likelihood of return would be small. 

As we have seen (Chapter I) Maupas found in a 
number of protozoa that if he picked out an individual 
(after each two divisions) to become the progenitor 
of the next generation, the rate of division after a 
time slowed down. ‘The individuals became weaker 
and finally the race died out. Calkins repeated the 
experiments with paramcecium on a larger scale and 
obtained similar results. The question arose whether 
the results were not due to the hay infusion lacking 
certain chemical substances that in time produced an 
injurious effect. Calkins tested this by transferring 
his weakened strains to different culture media. The 
result was that the race was restored to more than 
its original vigor. But very soon degeneration again 
set in. A new medium again restored vigor to some 
degree, but only for a short time, and finally the 
oldest culture died out in the 742d generation. It 
was evident, therefore, that if the slackened rate of 
division and other evidences of degeneration were in 
part due to the medium, yet some of the effects 
produced were permanent and could not be effaced by 
a return to a more normal medium. Then came 
Woodruff’s experiments. He kept his paramececia on 


pith Rie Y 199 


a mixed diet — on the kind of materials that it would 
be likely to meet with in nature, alternating with hay 
and other infusions. He found no degeneration, and 
at his last report his still vigorous strain was in’ the 
3000th generation. 

How can we harmonize these different results? 
It is hazardous, perhaps, to offer even suggestions, but 
if we assume that in a medium not properly balanced 
paramoecium is likely to degenerate in the sense that it 
loses some of its hereditary factors, we can understand the 
failure to become normal when this has once taken place 
even in a new environment. ‘Temporarily the decrepit 
individual may be benefited by a change, but not per- 
manently if its hereditary mechanism is affected. In 
Woodruff’s experiment the normal environment brings 
about no degenerative changes in the hereditary mech- 
anism and the race continues to propagate indefinitely. 

Let us turn now to the other side of the question 
and see what results cross-fertilization has given. 

Hyde has found that if two strains of flies with low 
fertility are crossed, there 1s a sudden increase in the 
output, as seen in the diagram (Fig. 96). The facts 
show clearly an improvement. More eggs of each 
strain are fertilized by sperm from the other strain 
than when the eggs are fertilized by sperm from the 
same strain.! In this case the results are not due 
to a more fertile individual being produced (although 
this may be true) but to foreign sperm, acting better 
than the strain’s own sperm. The evidence, as such, 
does not show whether this is due to each strain having 
degenerated in certain directions, or to some other 
kind of a change in the heredity complex. 


200 HEREDITY AND SEX 


The egg counts show that in the inbred stock many 
of the eggs are not fertilized, or if fertilized (82%) 
they still fail to develop. This means a decrease 
in fertility in the sense in which that word is here 
used. The offspring that arise from the cross-fer- 
tilization of these strains are more vigorous than their 
parents, if their increased fertility be taken as the 
measure of their vigor. The latter result is not shown 
in the table, for here 52% and 58% are the percent- 
ages of fertile eggs produced when the two strains are 
crossed. 


_Hislory of Inbred Sfock._ 


Fi. "2 | 3 4) NS Ge 7 6-8. 9" HO ieee 


368. 209 19) 184 @ #9 - -—- - = = = 156 
Cross of Fiz by Truncale 
Truncated by Truncale d 24% Fi3s =FygQ 32% 





5R% 58% 


Fia. 96. — The horizontal line F:—-F\3 gives the average number of flies 
per pair that emerged from inbred stock, decreasing from 368 to 156 per pair. 
Below is shown the results of a cross between a race of Truncates (short 
wings) and Fis. The percentages here give the number of eggs that hatched 
in each case. 


Darwin found that cross-fertilization was bene- 
ficial in 57 species of plants that he studied. In the 


1'The upper line F;—F; gives the average output of flies per pair. 
Below this line the percentages mean the number of isolated eggs 
that hatched. 


FERTILITY 


201 


case of primula, which is dimorphic, he found not only 
that self-fertilization gave less vigorous plants, but 
that when pollen from a long-styled flower of one plant 
fertilizes the pistil of another long-styled plant the 
vigor of the offspring is less than when the same kind 
of pollen is used to fertilize the pistil of a short-styled 


flower. 


The next table gives the detailed results. 





















































Maximum | Minrimum | AVERAGE 
NUMBER OF | NUMBER % 
OF SEEDSIN|OF SEEDSIN| NO. OF 
NATURE OF UNION FLOWERS OF SEED 
Any ONE | ANy ONE | SEEDS PER 
FERTILIZED | CAPSULES 
CAPSULE CAPSULE CAPSULE 
Long-styled form by 
pollen of — short- 10 6 62 34 46.5 
styled form: 
Legitimate union. 
Long-styled form by 
own-form pollen: 20 4 49 2 20.0 
Illegitimate union. 
Short-styled form by 
Be eg 10 8 61 37 47.7 
styled form: 
Legitimate union. 
Short-styled form by 
own-form pollen: 17 3 19 9 12.1 
Illegitimate union. 
The _two legitimate 20 14 62 34 47.1 
unions together. 
The two illegitimate 37 - 49 9 21.0 


unions together. 


























We know now that these two types of plants — long- 
styled and short-styled — differ from each other by 


a single Mendelian factor. 


We may therefore state 


202 HEREDITY AND SEX 


Darwin’s result in more general terms. The hetero- 
zygous plant is more vigorous than the homozygous 
plant. Moreover, in this case it is not the presence 
of the dominant factors that makes greater vigor (for 
the short-styled plant containing both dominants is 
less vigorous than the heterozygous), but the presence 
of two different factors that gives the result. 








At left of figures there are two strains of pure bred corn and 


HIG. 97. 
at right the hybrids produced by crossing those two pure strains. (After 
East.) 


The most thoroughly worked out case of the effects 
of inbreeding and cross-breeding is that of Indian corn. 
In recent years East and G. H. Shull have studied on 
a very large scale and with extreme care the problem 
in this plant. Their results are entirely in accord on 
all essential points, and agree with those of Collins, 
who has also worked with corn. 

Kast and Shull find that when two strains of corn 


vy 


sid oh oO 6 ey Mal ba 203 


(that have been to a large extent made pure) are crossed, 
the offspring is more vigorous than either:parent (Fig. 





Fig. 98. — At left an ear of Leaming Dent corn, and another at right 
after four years of inbreeding. The hybrid between the two is shown in the 
middle ear. (After East.) 


97). This is clearly shown in the accompanying pie- 
tures. Not only is the hybrid plant taller and stronger, 
but in consequence of this, no doubt, the yield of corn 


204 HEREDITY AND SEX 


per bushel is much increased, as shown in the next 
figure (Fig. 98). 

When the vigorous F; corn 1s self-fertilized, it produces 
a very mixed progeny, more variable than itself. Some 
of the F, offspring are like the original grandparental 
strains, some like the corn of first generation, and 
others are intermediate (Fig. 99). 


4 No. Noid, CAXD F, 
ei, 7 bu peracre 46.6 bu perocre : HAS bu. peracre: 





Fie. 99. — No. 9 and No. 12, two inbred strains of Leaming Dent corn 
compared with F; and F2 (to right). (After East.) 


It will not be possible for us to go into an analysis 
of this case, but Shull and East have shown that the 
results are in full harmony with Mendelian principles 
of segregation. The vigor of the fF; corn is explained 
on the basis that it is a hybrid product. To the extent 
to which the two parent strains differ from each other, 
so much the greater will be the vigor of the offspring. 

This seems an extraordinary conclusion, yet when 
tested it bears the analysis extremely well. 

Shull and apparently East also incline to adopt the 


FERTILITY 205 


view that hybridity or heterozygosity itself is the basis 
for the observed vigor; but they admit that another 
interpretation is also possible. For instance, each of 
the original strains may have been deficient in some of 
the factors that go to make vigor. Together they give 
a more vigorous individual than themselves. 

Whitney ran one line of hydatina through 384 par- 
thenogenetic generations, when it died (Line A). An- 
other line was carried through 503 generations, and at 
the last report was in a very weakened condition (Line 
B). When the former line was becoming extinct, he 
tried inbreeding. From the fertilized eggs he ob- 
tained a new parthenogenetic female. It showed 
scarcely any improvement. The other line gave similar 
results. In one case he again inbred for a second time. 
He found that the rates of reproduction of lines A and 
B were scarcely, if at all, improved. 

Whitney then crossed lines A and B. At once an 
improvement was observed. The rate of reproduction 
(vigor) was as great as that in a control line (reared 
under the same conditions) that had not deteriorated. 

The experiments of A. F. Shull on hydatina were 
somewhat different. He began with the twelfth gen- 
eration from a sexual egg. The line was supposedly 
not in a weakened condition. He inbred the line and 
obtained from the fertilized egg a new parthenogenetic 
series. After a few generations he inbred again. The 
results are shown in the next table. It is clear that 
there has been a steady decline despite sexual repro- 
duction, measured by four of the five standards that 
Shull applied, namely, size of family of parthenogenetic 
females, and of sexual females, number of eggs per day, 


206 HEREDITY AND SEX 


SHOWING DECREASE OF V1IGOR, AS MEASURED BY VARIOUS CHAR- 
ACTERS, IN Stx SuccESSIVELY INBRED PARTHENOGENETIC LINES 
oF Hydatina senta 















































NUMBER OF PARTHENOGENETIC 
nD LINE 
a CHARACTER TO BE MEASURED 
on 
a 1 2 3 4 5 6 
I. | Size of family of parthenogenetic female . . |48.4 142.5 |46.8 142.5 131.0 |22.6 
Size of family of fertilized sexual female . . (16.7 |12.8 |12.8 |11.5 | 6.3 | 7.3 
Number of eggs laid perday . . . » 5 (L1.0.111.4- 110-3) 100 
Number of days required to reach maturity | 2.27| 1.66) 2.25) 1.93 2.25] 2.12 
Proportion of cases in which first daughter | 
aid not become parente + a) a.) sn een Lyd | 1/3 | 2/4 |3/16| 0/4 | 5/8 
HamMerin Percentages! une, here Ce ee 14.2 25.0 41.6 
II. | Size of family of parthenogenetic female . . |48.4 |30.8 |41.0 |37.0 |33.8 |24:8 
Size of family of fertilized sexual female . . ,16.7 |13.7 |13.5 (15.2 LOE 177.6 
Number of eggs laid perday .. . 11.07 | L1.6 FeO Fez eo OG 
Number of days required to reach mat Seat PPA Mays, pe 2.20} 1.90} 2.00 
| Proportion of cases in which first daughter | 
did:notibecome:parent i) ose Se LY arg 2/7 | 2/10 8/20 | 7/16 
Same in percentages 25.0 23. 5 41.6 

















number of times the first daughter was too weak to 
become the mother of a new line. It is clear that 
inbreeding did not lead to an increase in vigor. 

In paramcecium there is also some new evidence. 
Calkins in 1904 brought about the conjugation of two in- 
dividuals of a weak race in the 354th generation. From 
one of the conjugants a new line was obtained that 
went through another cycle of at least 376 generations 
in culture, while during the same time and under sim- 
ilar conditions the weakened race from which the con- 
jugants were derived underwent only 277 generations. 

Jennings has recently reported an experiment in 
which some paramoecia, intentionally weakened by 
breeding in a small amount of culture fluid, were 


FERTILITY 207 


allowed to conjugate. Most of the lines that descended 
from several pairs showed no improvement but soon 
died out. In only one case was an individual produced 
that was benefited by the process. 

Jennings’ results are, however, peculiar in one very 
important respect. He did not use a race that had run 
down as a result of a long succession of generations, but 
a race that he had weakened by keeping under poor 
conditions. We do not know that the result in this 
case is the same as that in senile races or inbred races 
of other workers. It is not certain that the hereditary 
complex was affected in the way in which that complex 
is changed by inbreeding. He may have injured some 
other part of the mechanism. 

Jennings interprets conjugation in paramoecium to 
mean that a recombination of the hereditary factors 
~ takes place. Some of these combinations may be more 
favorable for a given environment than are others. 
Since these will produce more offspring, they will soon 
become the predominant race. 

The next diagram (Fig. 100) will serve to recall the 
principal facts in regard to conjugation in paramce- 
cium. ‘Two individuals are represented by black and 
white circles. At the time of conjugation the small 
or micronucleus in each divides (B), each then divides 
again (C). Four nuclei are produced. One of these 
micronuclei, the one that les nearest the fusion point, 
divides once more, and one of the halves passes into the 
other individual and fuses there with another nucleus. 
The process is mutual. Separation of the two indi- 
viduals then takes place and two ex-conjugants are 
formed. Each has a new double nucleus. This nu- 


208 HEREDITY AND SEX 


cleus divides (@) and each daughter nucleus divides 
again (H), so that each ex-conjugant has four nuclei. 


A = 
B = = 
G SBSFEee e& 
D Bp 
S E 7 
= F = 
{7 
oe is CS ee 
= = G = = 
= = = = asbet = : 5 = 
ye 6 6 EEC 6 eé Se & & 646 eé = 
PSCECCSCC?CTCSCST ! SCSCRECSESC ES z 
“a o gy GQ Pa Sy. GQ DS 
> € &€ €- E€E4€E€€E 
Fic. 100. — Diagram to show the history of the micronuclei of two 


Parameecia during (A-F) and after (F—J/) conjugation. Compare this dia- 
gram with Fig. 2. 


Another division gives eight nuclei in each. ‘The para- 
moecium itself next divides —each half gets four nuclei. 
A second division takes place, and each gets two of 
the nuclei. Four new individuals result. In each of 


FERTILITY 209 


these individuals one of the nuclei remains small and 
becomes the new micronucleus, the other enlarges to 
form the new macronucleus. Thus from each ex- 
conjugant four new paramoecia are produced, which 
now proceed to divide in the ordinary way, 1v.e. the 
micronucleus and the macronucleus elongate and divide 
at each division of the animal. 

It is customary to regard some phase in this process 
as Involving a reduction division in the sense that a 
separation of the paired factors takes place. If this 
occurs prior to interchange of micronuclei (/), then each 
ex-conjugant corresponds to an egg after fertilization. 
It is conceivable, however, that segregation might oc- 
cur in the two divisions that follow conjugation, which 
would give a different. interpretation of the process 
than the one followed here. 

On the first of these two hypotheses two new strains 
result after conjugation. Each is a recombination of 
factors contained in the two parents. If the two par- 
ents were alike, 7.e. homozygous, in many factors, and 
different, 7.e. heterozygous, in a few, the two individuals 
would be more alike than were the original races from 
which they came. ‘This is, in fact, what Jennings has 
shown to be the case, at least he has shown that on 
the average the ex-conjugants are more like each other 
than were the original strains. 

Calkins has obtained some new and important facts 
concerning the likeness and unlikeness of the new 
strains that result from conjugation. He has used 
wild, z.e. not weakened, individuals, and has followed 
the history of the four lines resulting from the first 
four individuals produced by each ex-conjugant. The 


210 HEREDITY AND SEX 


history of six such ex-conjugants is shown in the next 
diagram (Fig. 101). The four lines, ‘‘ quadrants,” 
(1, 2, 3, 4) that are descended from each of six ex- 
conjugants (viz. G, H, L, M, Q, B) are shown. At 
intervals large numbers of the populations were put 
under conditions favorable to conjugation and the 


Fayugalion variations tr Stix tx-coryuguntls of Sarumeeium caudadum 


Deée Dec Dee rs Pa Pairs Patrs DPD y 
| ape June tet Feb pest (nat rest Ca of ‘est # “ nie ters A at was ae pt st” 
= Q 4.9—46I0 if 
hos ™is O x | ia 3 
G Serces} i Z 
#2113. rae O59 od (4ft3 








IG = 914 5 =< 4 











re 











2 
©) 





























Fig. 101. — History of six (G, H, L, M, Q, B) ex-conjugants. In each 
the descendants of the first four individuals (after conjugation) is shown; 
the numbers indicate the pairs of conjugants counted when the test was 


made. X indicates deaths; O indicates that no conjugation took place. 
(After Calkins.) 


number of conjugating pairs counted. The results 
are shown in the diagram. The circles indicate no 
conjugations; XY indicates the death of the strain. 
In the G and in the M series many conjugations took 
place. In other series conjugation did not take place 
until much later. Striking differences appear in the 
different quadrants although they were kept under 
similar conditions. 


FERTILITY 211 


But even amongst the four lines descended from the 
same ex-conjugant marked differences exist. These 
differences cannot be attributed to constitutional dif- 
ferences unless a segregation of factors takes place 
after conjugation or unless it can be shown that these 
differences are not significant. In the light of these 
conflicting results on paramoecium it may seem unsafe 
to draw any far-reaching conclusions concerning the 
nature of sexual reproduction in general from the evi- 
dence derived from these forms. In the higher animals, 
however, the evidence that segregation takes place 
prior to fertilization and that recombinations result 
can scarcely be doubted. 


THEORIES OF FERTILITY 


Let us now try to sum up the evidence in regard to 
the influence of cross-fertilization. This can best be 
done by considering the three most important hypoth- 
eses that have been brought forward to explain how 
crossing gives greater vigor. 

Shull and East explain the vigor of the hybrid by 
the assumption that it contains a greater number of dif- 
ferent factors in its make-up than either of its parents. 
They support the view by an appeal to the next (/2) 
generation from such hybrids that shows a lower 
range of vigor, because, while a few individuals of this 
generation will be as mixed as the hybrid (/1), and 
therefore like it, most of them will be szmpler in com- 
position. This interpretation is also supported by the 
evidence that when pure lines (but not necessarily, 
however, homozygous lines) are obtained by self-fer- 
tilizing the offspring of successive generations from 


212 HEREDITY AND SEX 


these first hybrids, further decline does not take 
place. 

An alternative view, that is also Mendelian, has been 
offered by Bruce and by Keeble and Pellew. Vigor, it 
is maintained, is in proportion to the number of domi- 
nant factors, and in proportion to the number of these 
factors present whether in a hybrid or in a homozygous 
(duplex) condition. 

On this view the hybrid is vigorous, not because it is 
hybridous, so to speak, but because in its formation a 
larger number of dominant factors (than were pres- 
ent in either parent) have been brought together. 

A third view is also compatible with the evidence, 
namely, that there may exist factors that are them- 
selves directly concerned with fertility. There is one 
such case at least that has been thoroughly analyzed 
by Pearl. 

Pearl studied for five years the problem of fertility 
in two races of fowls, viz. barred Plymouth rocks and 
Cornish Indian games. The main features of his 
results are shown in the diagram (Fig. 102). He finds 
that the winter output of eggs, which is correlated 
with the total production, is connected with two factors. 
One factor, designated by L, is a non-sex-linked char- 
acter. Ifit is present, an average of less than 30 eggs 
is produced in the winter season. There is another 
factor, L2, that is present in the barred rocks, but not — 
in the Indian game. If present alone, the winter out- 
put is again about 30 eggs on an average. If, how- 
ever, both LZ, and ZL, are present, the winter output 
is more than 30 and may be as great as 90, or in rare 
cases 100-120 eggs. 


FERTILITY 213 


The peculiarity about this discovery is that the 
second factor, Lz, is sex-linked, which means in this case 
that it is carried by the eggs that will produce the males 
in the next generation, and not by the eggs that will 
produce the daughters. Hence if the daughters of high- 
producing hens are selected, one does not get in them 


Inherilance of Ferfilify in Fowl. (Pearl) 





Low? fF.L,—L, Flea—Leg Low? 
(Zero od) 4,— Ly Lg— lz Zerd 
FLO Low File te Zero? 
Lilt (Low) hele (Low)d 

hats oerreaaad Lot, (Low) J 

FL, 4abet, High 

L, tz het, High)c 





3 ta ty (Low) o 
i, Q Fle how fe) 
eae Uh (High) o 
pate Fi Ne 
s ff Le— te 
FL, High? tah, EHighd 
eet Obie, to (Lows 


Fig. 102. — Illustrating Pearl’s hypothesis. F = female factor present 
in half of the eggs and determining sex. JL, = factor for low egg produc- 
tion; i, its allelomorph for zero production of winter eggs. IL» = factor 
for high winter production; /:, its allelomorph. 





the high productiveness of the mother. It is her sons 
that inherit the character, although they cannot show 
it except in their offspring. 

Aside from whatever practical interest these results 
may have, the facts are important in showing that such 
a thing as a factor for fertility itself may be present, 
without otherwise being apparent, and that this factor 


214 HEREDITY AND SEX 


taken in connection with another (or others) gives high 
productivity. 

The other point to which I wish to call attention 
relates to a different matter. We have met with some 
eases where lowered fertility was due to eggs failing 





\ 


Fig. 103. — Normal male of Drosophila (on left) and male with ‘‘rudi- 
mentary’’ wings (on right). Note sex comb (lower left). 


to a greater or less degree to be fertilized by sperm of 
the same strain. 

A striking case of this kind is found in a mutant of 
the fruit fly that appeared in my cultures. The mu- 
tant has rudimentary wings (Fig. 103). The females 
are absolutely infertile with males of the same kind. 


FERTILITY 215 


If they are mated to any other male of a different strain, 
they are fertilized. The males, too, are capable of fer- 
tilizing the eggs of other strains, in fact, are quite 
fertile. 

The factor that makes the rudimentary winged 
fly is of such a sort that it carries infertility along with 
it —in the sense of self-infertility. This result has 
nothing to do with inbreeding, and the stigma cannot 
be removed by crossing out and extracting. 

A somewhat similar factor, though less marked, is 
found by Hyde in certain of his inbred stock to which 
I have referred. As his experiments show, the infer- 
tility in this case is not due to lack of eggs or sperm, but 
to a sort of incompatibility between them so that not 
more than 20 per cent of the eggs can be fertilized by 
males of the same strain. 

In the flowering plants where the two sexes are often 
combined in the same individual, it has long been known 
that there are cases in which self-fertilization will not 
take place. The pollen of a flower of this kind if placed 
on the stigma of the same flower or of any other flower 
on the same plant will not fertilize the ovules. Yet the 
pollen will fertilize other plants and the ovules may be 
fertilized by foreign pollen. 

Correns has recently studied that problem and has 
arrived at some important conclusions. He worked 
with a common plant, Cardamine pretensis. In this 
plant self-fertilization is ineffectual. He crossed plant 
B with plant G, and reared their offspring. He tested 
these with each other and also crossed each of them back 
to its parents that had been kept alive for this pur- 
pose. The latter experiment is simple and more in- 


216 HEREDITY AND SEX 


structive. His results and his theory can best be 
given together. 

Correns assumes that each plant contains some factor 
that produces a secretion on the stigma of the flowers. 
This secretion inhibits the pollen of the same plant 
from extending its pollen tube. He found, in fact, 
that the pollen grains do not grow when placed on the 
stigma of the same plant. All plants will be hybril 


Back Cross tn Cardamine pidindst (Conc) 


OE Boat 

Cane Gang 
BG oe or 
B — 3 
Fg gp Be 
hg ) + at 


Fig. 104. — Illustrating the crossing of the types Bb and Gg to give four 
classes: BG, Bg, bG, bg. Each of these is then back-crossed either to B or 
to G with the positive (+) or negative (—) results indicated in the diagram. 
for these factors, hence plant B will produce two kinds 
of germ-cells, B and b. Similarly, plant G will produce 
two kinds of germ-cells, G-g. If these two plants are 
crossed, four types will be produced. When these are 
back-crossed to the parents, the expectation is shown in 
the diagram (Fig. 104). Half the combination should 
be sterile and half should be fertile. This is, in fact, 
what occurs, as shown in the same diagram. ‘The 
— signs indicate that fertilization does not occur, while 
the + signs indicate successful fertilization. 

Correns’ theory is also in accord with other com- 


FERTILITY 217 


binations that he made. There can be little doubt 
that he has pointed out the direction in which a solu- 
tion is to be found. 

There is a somewhat similar case in animals. In one 
of the Ascidians, Ciona intestinalis, an hermaphrodite, 
the sperm will not fertilize the eggs of the same indi- 
vidual. But the sperm will fertilize eggs of other 
individuals, and vice versa. Castle first found out this 
fact, and I have studied it on a large scale. The 
diagram (Fig. 105) gives an example of one such ex- 
periment made recently by W. 8. Adkins. 

Five individuals are here used. The eggs of one. 
individual, A, were placed in five dishes (horizontal 
line); likewise those of 6, C, D, EH. The sperm of A, 
designated by a (vertical lines) was used to fertilize 
the eggs, A, B, C, D, EH; likewise the sperm 0, c, d, e. 
The self-fertilized sets form the diagonal line in the 
diagram and show no fertilization. The other sets 
show various degrees of success, as indicated by the 
percentage figures. These results can best be under- 
stood, I think, by means of the following hypoth- 
esis. The failure to self-fertilize, which is the main 
problem, would seem to be due to the similarity in the 
hereditary factors carried by eggs and sperm; but 
in the sperm, at least, reduction division has taken 
place prior to fertilization, and therefore unless each 
animal was homozygous (which from the nature of the 
case cannot be assumed possible) the failure to fertilize 
cannot be due to homozygosity. But both sperm and 
eggs have developed under the influence of the total 
or duplex number of hereditary factors; hence they 
are alike, 2.e. their protoplasmic substance has been 


218 HEREDITY AND SEX 


under the same influences. In this sense, the case is 
like that of stock that has long been inbred, and has 


SUE and Cross Fertilization i Gore. 





Fic. 105. — The oblique line of letters A%, B®, C°, D4, E®, gives the self- 
fertilized sets of eggs; the rest A?, A°, etc., the cross-fertilized sets. A, B, 
C, D, E = eggs; a, b, c, d, e, = sperm of same individuals. (From unpub- 
lished work of W. S. Adkins.) 


come to have nearly the same hereditary complex. if 
this similarity decreases the chances of combination be- 
tween sperm and eggs, we can interpret the results. Cor- 
rens’ results may come under the same interpretation. 


FERTILITY 219 


I have tried to bring together the modern evidence 
that bears on the problems of fertility and sterility. 
It is evident that there are many obscure relations that 
need to be explained. I fear that, owing to the diffi- 
culty of summarizing this scattered and diverse ma- 
terial, I have failed to make evident how much labor 
and thought and patience has been expended in ob- 
taining these results, meager though they may appear. 

But while it is going to take a long time and many 
heads and hands to work out fully these problems, there 
can be little doubt that the modern method is the only 
one by which we can hope to reach a safe conclusion. 


CHAPTER VIII 
SPECIAL CASES OF SEX-INHERITANCE 


THE mechanism of sex-determination that we have 
examined gives equal numbers of males and females. 
But there are known certain special cases where equality 
does not hold. I have selected six such cases for 
discussion. Each of these illustrates how the mechan- 
ism of sev-determination has changed to give a different 
result ; or how, the mechanism remaining the same, some 
outside condition has come in that affects the sex ratio. 

It is so important at the outset to clearly recognize 
the distinction between sex-determination and _ sex 
ratio, that I shall take this opportunity to try to make 
clear the meaning of this distinction. The failure to 
recognize the distinction has been an unfailing source 
of misunderstanding in the literature of sex. 

(1) A hive of bees consists of a queen, thousands of 
workers, and at certain seasons a few hundred drones 
or males. The workers are potentially females, and 
these with the queen give an enormous preponderance 
of females. In this case the explanation of the sex 
ratio is clear. Most of the eggs laid by the queen are 
fertilized, and in the bee all fertilized eggs become fe- 
males, because as we have seen there is only one class of 
spermatozoa produced, and not two as in other insects. 

There is a parallel and interesting case in one of the 
wasps described by Fabre. The female lays her eggs 

220 


SPECIAL CASES OF SEX-INHERITANCE 221 


as a rule in the hollow stems of plants, each egg in a 
separate compartment. In some of the compartments 
she stores away much more food than in others. From 
these compartments large females hatch. From com- 
partments where less food is stored the smaller males 
are produced. It may seem that the amount of food 
stored up determines the sex of the bee. To test this 
Fabre took out the excess of food from the large 
compartments. The wasp that emerged, although 
small for want of food, was in every case a female. 
Fabre enlarged the smaller compartments and added 
food. The wasp that came out was a male, larger 
than the normal male. 

It is evident that food does not determine the sex, 
but the mother wasp must fertilize the eggs that she 
lays in chambers where she has stored up more food, 
and not fertilize those eggs that she deposits in com- 
partments where she has accumulated less food. 

(2) A curious sex ratio appeared in one race of fruit 
flies. Some of the females persisted in producing twice 
as many females as males. This was first discovered 
by Miss Rawls. In order to study what was taking 
place, I bred one of these females that had red eyes to 
a white-eyed male of another stock. All the offspring 
had red eyes, as was to be expected. I then bred these 
daughters individually to white-eyed males again 
(Fig. 106). Half of the daughters gave a normal 
ratio; the other half gave the following ratio: 


Red | Red White White 
4 3 3 
50 0 50 50 


222 HEREDITY AND SEX 


It is evident that one class of males has failed to ap- 
pear — thered males. If we trace their history through 
these two generations, we find that the single sex chro- 





% ? fe) 


Fic. 106. — Diagram to show the heredity of the lethal factor (carried 
by black X). <A, red-eyed female, carrying the factor in one X, is bred to 
normal white-eyed male. 8B, her red-eyed daughter, is bred again to a normal 
white-eyed male, giving theoretically the four classes shown in C, but one of 
the classes fails to appear, viz. the red-eyed male (colored black in the dia- 
gram). The analysis (to right) shows that this male has the fatal X. One 
of his sisters has it also, but is saved by the other X. She is the red-eyed 
female. If she is bred to a white-eyed male, she gives the results shown in 
D, in which one class of males is again absent, viz. the red-eyed male. In 
this diagram the black X represents red eyes and lethal (as though completely 
linked). 


SPECIAL CASES OF SEX-INHERITANCE 223 


mosome that each red male contains is one of the two 
chromosomes present in the original red-eyed grand- 
mother. If this chromosome contains a factor which 
if present causes the death of the male that contains it, 
and this factor is closely linked to the red factor, 
the results are explained. AIl the females escape the 
fatality, because all females contain two sex chromo- 
somes. If a female should contain the fatal factor, 
her life is saved by the other, normal, sex chromosome. 
The hypothesis has been tested in numerous ways and 
has been verified. We keep this stock going by mat- 
ing the red females to white males. This gives con- 
tinually the 2:1 ratio. The white sisters, on the other 
hand, are normal and give normal sex ratios. 

(3) Another aberrant result, discovered by Mr. 
Bridges, is shown by a different race of these same fruit 
flies. It will be recalled that when an ordinary white- 
eyed female is bred to a red-eyed male all the sons have 
white eyes. But in the race in question a different re- 
sult follows, as shown by the diagram. From 90 to 
95 per cent of the offspring are regular, but 5 per cent 
of the females and 5 per cent of the males are uncon- 
formable, yet persistently appear in this stock. 

The results can be explained if we suppose that the 
two sex chromosomes in the egg sometimes stick to- 
gether (Fig. 107). They will then either pass out into 
one of the polar bodies, in which case the red-eyed males 
will develop if the egg is fertilized by a female-producing 
sperm; or the two sex chromosomes will both stay in 
the egg, and give a kind of female with three sex chro- 
mosomes. 

Here also numerous tests can be made. They verify 


224 HEREDITY AND SEX 


the expectation. Thus by utilizing sex chromosomes 
that carry other sex-linked characters than white eyes, 
it can be shown that the results are really due to the 
whole sex chromosome being involved, and not to 
parts of it. The result is of unusual interest in another 
direction; for it shows that the female-producing 


Son separation Bult s) 
K— K— KK— OO 
X= 0 


BRB XY xe | 95 M(()) mea} 
WH) XOC ere! SOO mas) s 
X0O0~ss OOO 


Fria. 107. — Non-disjunction of the sex chromosomes. In consequence 
a female produces three instead of one class of eggs (see to right of diagram) 
with respect to X. The results of the fertilization of such a female by 
a normal red male are shown in the lower part of the diagram. 








WO, 


sperm will make a male if it enters an egg from which 
both sex chromosomes have been removed. It is 
therefore not the female-producing sperm, as such, 
that gives a female under normal conditions, but this 
sperm plus the sex chromosome already present in 
the egg that gives an additive result — a female. 

(4) In the group of nematode worms belonging es- 
pecially to the genus Rhabditis, there are some extraor- 


SPECIAL CASES OF SEX-INHERITANCE 225 


dinary perversions of the sex ratios. The table gives 
the ratios that Maupas discovered. Not only are the 





Diplogaster robustus . . . . . . 0.138 male 
mhopoiisruienarai’ . ts. . ..). «60.15 male 
Pisneiesduienura: +. <*..4 .. O'7 emale 

Rhabditis caussaneli . . . . . . 1.4 males 

PRUAMCIIteVAUS. vee cfs 2 62). / 1.55, males Eg 1000 famines 
heaedinrsreorondata..). 2... 4.0 5.0) males 

Rusodiae perrierie.\. . 7s . . »« 7.0° males | 

PO eee Marion sas. <i. 3 . 7.6 males || 
Riaabdideauthiers! «=. (». 1. ..20:0 males 
Riapoinsvigiwiert ..).7. ..: .45.0 males 


males extremely rare — almost reaching a vanishing 
point in certain cases — but they have lost the instinct 
to fertilize the female. 

The females, on the other hand, have acquired the 
power of producing sperm, so that they have passed 
over into the hermaphroditic state. The behavior 
and history of the sperm that the females produce has 
only recently been made out by Miss Eva Kriiger. 
It is found that a spermatozo6n enters each egg and 
starts the development, but takes no further part in 
the development (Fig. 108). The egg may be said to 
be half fertilized. It is a parthenogenetic egg and 
produces a female. 

(5) Some very high male ratios have been reported 
by Guyer in cases where birds of very different families 
have been crossed — the common fowl by the guinea 
hen, individuals of different genera of pheasants bred 
to each other and to fowls, ete. Hybrids between 
different genera gave 74% —13?. Hybrids between 
different species of the same genus 72¢ —18?. In 
most of these cases, as Guyer points out, the sex is 


226 HEREDITY AND SEX 


recorded from the mounted museum specimen which 
has the male plumage. But it is known that the re- 
productive organs of hybrids, extreme as these, are gen- 
erally imperfect and the birds are sterile. It has been 


Fig. 2. 
- Fig. 3. 


re 
ae Sash rs 








Fic. 108. — Odgenesis and spermatogenesis of habditis aberrans. 
1-5, stages in odgenesis, including incomplete attempt to form one polar 
body. Eighteen chromosomes in 1 and again in 4 and 5. In3 the entering 
sperm seen at right. 6, prophase of first spermatocyte with 8 double and 
two single chromosomes (sex chromosomes). At the first division (7) the 
double chromosomes separate, and the two sex chromosomes divide, giving 
ten chromosomes to each daughter cell (8). At the next division the two 
sex chromosomes move to opposite poles, giving two female-producing 
sperm (9 and 10). Rarely one of them may be left at the division plane 
and lost, so that a male-producing sperm results that accounts for the rare 
occurrence of males. (After E. Kriger.) 


shown that if the ovary of the female bird is removed 
or deficient, she assumes the plumage of the male. 
Possibly, therefore, some of these cases may fall under 
this heading, but it is improbable that they can all be 
explained in this way. In the cases examined by Guyer 
himself the hybrids were dissected and all four were 
found to be males. 


SPECIAL CASES OF SEX-INHERITANCE 220 


Pearl has recently pointed out that the sex ratio 
in the Argentine Republic varies somewhat accord- 
ing to whether individuals of the same race, or of dif- 
ferent races, are the parents. As seen in the following 
table, the sex ratio of Italian by Italian is 100.77 ; 


CoMPARISON OF THE SEX RATIOS OF THE OFFSPRING OF PURE AND 
Cross MaATINGS 






































MatTINGS Sex Ratio Oe ANGE 
Italian ¢ Argentine @ 105.72 +.46 
Italian ¢ Italian ? 100.77 +.20 
Difference 4.95 + .50 9.9 
ee are, a 
Italian ¢ Argentine ? 105.72 + .46 
Argentine g Argentine 9 103.26 + .34 
Difference 2.46 +.57 4.3 
nes Bb a lt 
Spanish ¢@ Argentine ? 106.69 + .74 
Spanish ¢ Spanish 105.55 +.36 
Difference 1.14 +.82 1.4 
a amen 
Spanish ¢ Argentine ° 106.69 +.74 
Argentine @ Argentine 2 103.26 + .34 
Difference 3.43 +.81 4.2 














Argentine by Argentine, 103.26 ; but Italian by Argen- 
tine, 105.72. If, as has so often been found to be the 
case, a hybrid combination gives a more vigorous 
progeny, the higher sex ratio of the cross-breed may 
account for the observed differences, since other data 
show that the male infant is less viable and the in- 
ereased vigor of a hybrid combination may increase 
the chance of survival of the male. 


228 HEREDITY AND SEX 


(6) We come now to the most perplexing case on 
record. In frogs the normal sex ratio is approximate 
equality. Professor Richard Hertwig has found that 
if the deposition of the eggs is prevented for two to 
three days (after the eggs have reached the uterus) 
the proportion of males is enormously increased — 
in the extreme case all the offspring may be males. 
By critical experiments Hertwig has shown that the 
results are not due to the age of the spermatozoa, al- 
though in general he is inclined to attribute certain 
differences in sex-determination to the sperm as well 
as to the eggs. 

The evidence obtained by his pupil, Kuschakewitsch, 
goes clearly to show that the high male sex ratio is 
not due to a differential mortality of one sex. 

In the following table four experiments (a, 6, c, d) 
are summarized. The interval between each record 


paler t 

a) ai ole 0 PagTs 

| ees. feee dS Bas. 

b) 834 9:47¢ Obie Cin 156 9: 194¢ 7 O248 3 
VN VAIN 

¢) 64 2: 61¢ LO lessee & 115 9: 169¢ 
Tapers ras Wie 

d) 55 9: 52¢ 148 9: 87¢ (Abies dies | Ry P29 bs" beg 


is written above in hours. In all cases an excess of 
males is found if the eggs have been kept for several 
hours before fertilization. In the first (a), second (b), 
and fourth (d) cases the excess of males is very great. 

Hertwig attempts to bring his results into line with 


SPECIAL CASES OF SEX-INHERITANCE 229 


his general hypothesis of nucleo-plasm relation. He 
holds, for instance, that sex may be determined by the 
relation between the size of the nucleus and the proto- 
plasm of the cell. As the value of the evidence has 
been seriously called into question in general, and as 
there is practically no evidence of any weight inits favor 
in the present case, I shall not dwell further on the 
question here. But the excessively high male ratio is 
evident and positive. How to explain it is difficult 
to say. Itis just possible, I think, that delay may have 
injured the egg to such an extent that the sperm may 
start the development but fail to fuse with the egg 
nucleus. Under these circumstances there is the possi- 
bility that all the frogs would be males. 

Miss King has also carried out extensive sets of ex- 
periments with toads and frogs. She has studied the 
eggs and the sperm under many different conditions, such 
as presence of salt solutions, acids, sugar solutions, cold, 
and heat. Her results are important, but their inter- 
pretation is uncertain. In sugar solutions and in dry 
fertilization she has increased the proportion of males 
to 114 per 1002. The normal sex ratio for the toad is 
903 to 1002. Whether the solutions have in any sense 
affected the determination of sex, or acted to favor 
one class of sperm at the expense of the other remains 
to be shown, as Miss King herself frankly admits. 

In the case of man there are extensive statistics 
concerning the birth rate. The accompanying tables 
give some of the results. There are in all parts of the 
world more males born than females. The excessively 
high ratios reported from the Balkans (not given here) 
may be explained on psychological grounds, as failure 


230 HEREDITY AND SEX 


MaA.LeEs 
LLG Vi, ck re ee ees 
Nrancey, i GM Cees s ca LO 450 
Ring land? st: ¥2 "Gece eel Uo 
(Ecce Nanay Creek. cy he Seat EO 
AUStTIO® ol. is Bl ee eee . 
Hungary .« ©): 20 & ~~ 105.032 to 0G terraie 
Switzerland] =o. #0. 7 eL045 
Beloit tins ope eee ae La 
Holland® =. .¢ulewe se em Uo 
Spaltest ic. ee. Awe ie ee Ses 
RUsSIase ovules wee eee ie Lee 


to report the birth of a boy is more likely to lead to the 
imposition of a fine on account of the conscription. 

There can be no doubt, however, that slightly more 
males than females are born. Moreover, if the still- 
born infants alone are recorded, surprisingly large ratios 
occur, as shown in the next table. 


MALES 
Ttaly-2.\ Se ee ee ok 
Erance:? >. fee eae eee ae 
Germany. ..: 40. o=1288 
AUSELIO 9h a ee ee eee 
Hungary 2) see ee OULU 
Switzerland. . . . .:. 135.0 } to 100 females 
Belriuim es. 1 pee lo 
Hollands 33-2 ete eee 
Sweden? oy ox 5s eee eee oo 
IN OFWAY fn se ce eee eG 
Denmark yi eee reat ta 


And if abortive births are also taken into account, the 
ratio is still higher. It seems that the male embryo 
is not so strong as the female, or else less likely, from 
other causes, to be born alive. 

In many of the domesticated animals also, especially 


SPECIAL CASES OF SEX-INHERITANCE 231 


the mammals, there is an excess of males at birth, as 
the next table shows. 


MaALrEs FEMALES 
frotseees. eee ke ik 98.31 100 (Diising) 
Pe etre ee oe O78 100 (Wilchens) 
SuCopE fee <.cPee a COTS. 100 (Irwin) 
Pigmeeeen ers. See L118 100 (Wilchens) 
Piette et ee LOO 100 (Cuénot) 
ereweete.. x. a J.e LOO:O 100 (Cuénot) 
Sema rant, ba as Pate OAL 7. 100 (Darwin) 


A little later I shall bring forward the evidence that 
makes probable the view that in man the mechanism 
for sex-determination is like that in other animals, 
where two classes of sperm are produced, male- and 
female-producing. How then can we account in the 
human race for the excess of eggs that are fertilized 
by male-producing spermatozoa? At present we do not 
know, but we can at least offer certain suggestions that 
seem plausible. 

In mammals the fertilization occurs in the upper 
parts of the oviduct. In order to reach these parts 
the sperm by their own activity must traverse a dis- 
tance relatively great for such small organisms. If 
the rate of travel is ever so slightly different for the two 
classes of sperm, a differential sex ratio will occur. 

Again, if from any cause, such as disease or alcoholism, 
one class of sperm is more affected than the other, a 
disturbance in the sex ratio would be expected. 

At present these are only conjectures, but I see 
no ground for seizing upon any disturbance of the 
ratio in order to formulate far-reaching conclusions 
in regard to sex-determination itself. As I pointed 
out in the beginning of this chapter, we may go 


232 HEREDITY AND SEX 


wide of the mark if we attempt to draw conclusions 
‘concerning the determination of sex itself from devia- 
tions such as these in the sex ratio, yet it is the mistake 
that has been made over and over again. We must 
look to other methods to give us sufficient evidence as 
to sex-determination. Fortunately we are now in a 
position to point to this other evidence with some 
assurance. With the mechanism itself worked out, 
we are in a better position to explain slight variations 
or variables that modify the combinations in this way 
or in that. 


THE ABANDONED VIEW THAT EXTERNAL CONDITIONS 
DETERMINE SEX 


But before taking up the evidence for sex-determina- 
tion in man I must briefly consider what I have been 
bold enough to eall the abandoned view that external 
conditions determine sex. 

Let us dismiss at once many of the guesses that have 
been made. Drelincourt recorded 262 such guesses, 
and Geddes and Thomson think that this number has 
since been doubled. Naturally we cannot consider 
them all, and must confine ourselves to a few that 
seem to have some basis in fact or experiment. 

The supposed influence of food has been utilized in 
a large number of theories. The early casual evidence | 
of lLandois, of Mrs. Treat, and of Gentry has 
been entirely set aside by the careful observations of 
Riley, Kellogg and Bell, and Cuénot. In the latter 
cases the experiments were carried through two or even 
three generations, and no evidence of any influence of 
nourishment was found. 


SPECIAL CASES OF SEX-INHERITANCE 2393 


The influence of food in sex-determination in man has 
often been exploited. It is an ever recurrring episode 
in the ephemeral literature of every period. The most 
noted case is that of Schenk. In his first book he said 
starvation produced more females; in his second book 
he changed his view and supposed that starvation 
produces more males. | 

Perhaps the most fertile source from which this view 
springs is found in some of the earlier statistical works, 
especially that of Diising. Diising tried to show that 
more girls are born in the better-fed classes of the com- 
munity, in the poorer classes more boys. The effective 
difference between these two classes is supposedly one of 
food! For instance, he states that the birth-rate for 
the Swedish nobility is 98 boys to 100 girls, while in the 
Swedish clergy the birth-rate is 108.6 boys to 100 girls. 

Other statistics give exactly opposite results. Pun- 
nett found for London (1901) more girls born amongst 
the poor than the rich. So many elements enter into 
these data that it is doubtful if they have much value 
even in pointing out causes that affect the sex ratio, and 
it is quite certain that they throw no light on the 
causes that determine sex. 

In other mammals where a sex ratio not dissimilar 
to that in man exists, extensive experiments on feeding 
have absolutely failed to produce any influence on 
the ratio. We have, for instance, Cuénot’s experi- 
ments with rats, and Schultze’s experiments with 
mice. The conditions of feeding and starvation were 
much more extreme in some cases than is likely to 
occur ordinarily, yet the sex ratio remained the same. 

Why in the face of this clear evidence do we find 


234 HEREDITY AND SEX 


zoOlogists, physicians, and laymen alike perpetually 
discovering some new relation between food and sex? 
It is hard to say. Only recently an Italian zoGlogist, 
Russo, put forward the view that by feeding animals 
on lecithin more females were produced. He claimed 
that he could actually detect the two kinds of eggs 
in the ovary — the female- and the male-producing. It 
has been shown that his data were selected and not 
complete; that repetition of his experiments gave no 
confirmative results, and probably that one of the two 
kinds of eggs that he distinguished were eggs about to 
degenerate and become absorbed. 

But the food theories will go on for many years to 
come — as long as credulity lasts. 

Temperature also has been appealed to as a sex fac- 
tor in one sense or another. R. Hertwig concluded 
that a lower temperature at the time of fertilization 
gave more male frogs, but -Miss King’s observations 
failed to confirm this. There is the earlier work of 
Maupas on hydatina and the more recent work of 
von Malsen on Dinophilus apatris. I have already 
pointed out that Maupas’ results have not been con- 
firmed by any of his successors. Even if they had been 
confirmed they would only have shown that tempera- 
ture might have an effect in bringing parthenogenesis 
to an end and instituting sexual reproduction in its 
stead. In hydatina the sexual female and the male 
producing individual are one and the same. A more 
striking case could not be found to show that the en- 
vironment does not determine sex but may at least 
change one method of reproduction into another. 

There remain von Malsen’s results for dinophilus, 


SPECIAL CASES OF SEX-INHERITANCE 235 


where large and small eggs are produced by the same 
female (Fig. 109). The female lays her eggs in clus- 
ters, from three to six eggs, as a rule, in each cluster. 
The large eggs produce females; the small eggs pro- 





Fia. 109. — Dinophilus gyrociliatus. Females (above and to left) and 
males (below and to right). Two kinds of eggs shown in middle of lower 
row. (After Shearer.) 


duce rudimentary males that fertilize the young fe- 
males as soon as they hatch and before they have left 
the jelly capsule. 

Von Malsen kept the mother at different tempera- 
tures, with the results shown in the table. The ratio 
of small eggs to large eggs changes. But the result 




















TEMPERATURE oars 3 2 aoe ante 
Room temp. 19°C. . 202 | 327 813 224 5,6 
(ibis Oh ae. 925 973 2975 1: 3,5 4,2 
Ine et 6 383 507 886 id Bye Bae a) 

















a 50) 


236 HEREDITY AND SEX 


obviously may only mean that more of the large eggs 
are likely to be laid at one temperature than at another. 
In fact, temperature seemed to act so promptly accord- 
ing to Von Malsen’s observations that it 1s very un- 
likely that it could have had any influence in deter- 
mining the kind of egg produced, but rather the kind 
of egg that was more likely to be laid. We may dis- 
miss this case also, I believe, as not showing that sex 
is determined by temperature. 


SEX-DETERMINATION IN MAN 


Let us now proceed to examine the evidence that 
bears on the determination of sex in man. I shall 
draw on three sources of evidence: 

1. Double embryos and identical twins. 

2. Sex-linked inheritance in man. 

3. Direct observations on the chromosomes. 

The familiar case of the Siamese twins is an example 
of two individuals organically united. A large series 
of such dual forms is known to pathologists. There 
are hundreds of recorded cases. In all of these both 
individuals are of the same sex, 2.e. both are males 
or both are females. There is good evidence to show — 
that these double types have come from a single fer- 
tilized egg. They are united in various degrees (Fig. 
110); only those that have a small connecting region 
are capable of living. These cases lead directly to 
the formation of separate individuals, the so-called 
identical twins. | 

Galton was one of the first, if not the first, to recognize 
that there are two kinds of twins — identical twins and 
ordinary or fraternal twins. 


SPECIAL CASES OF SEX-INHERITANCE 237 


Identical twins are, as the name implies, extremely 
alike. They are always of the same sex. There 
is every presumption and some collateral evidence 
to show that they come from one egg after fer- 
tilization. On the other hand, amongst ordinary 
twins a boy and a girl, or two boys and two girls, occur 
in the ratio expected, 7.e. on the basis that their sex is 


RRR ROR BA 
RRA AEKE 
NM yor de 06 


NH 6 eB 
\ Gs 


DIAGRAM SHOWING THE INTERRELATIONS OF THE VARIOUS SORTS OF DIPLOPAG! AND * 
‘DUPLICATE TWINS, ILLUSTRATIVE OF THE THEORY. ADVANCED IN THIS PAPER. FURTHER EX= 
PLANATION IN THE TEXT. 


Fig. 110. — Diagram showing different types of union of double monster 
(After Wilder.) 


not determined by a common external or internal 
cause. Since fraternal twins and identical twins show 
these relations at birth and from the fact that they 
have been in both cases subjected to the same condi- 
tions, it follows with great probability that sex in 
such cases is determined before or at the time of 
fertilization. : 

This conclusion finds strong support from the condi- 


238 HEREDITY AND SEX 


tions that have been made out in the armadillo. 
Jehring first reported that all the young of a single 
litter are of the same sex (Fig. 111). The statement 
has been verified by Newman and by Patterson on a 
large scale. In addition they have found, first, that 
only one egg leaves the ovary at each gestative period ; 
and second, that from the egg four embryos are pro- 





Fig. 111. — Nine-banded Armadillo. Four identical twins with a 
common placenta. (After Newman and Patterson.) 


duced (Fig. 112). The material out of which they 
develop separates from the rest of the embryonic 
tissue at a very early stage. The four embryos are 
identical quadruplets in the sense that they are more 
like each other than like the embryos of any other 
litter, or even more like each other than they are to 
their own mother. 

The second source of evidence concerning sex-deter- 


SPECIAL CASES OF SEX-INHERITANCE 239 


mination in man is found in the heredity of sex-linked 
characters. 

The following cases may well serve to illustrate 
some of the better ascertained characters. The tables 
are taken from Davenport’s book on ‘“‘ Heredity in 
Relation to Eugenics.’ The squares indicate males, 
affected males are black squares ; the heavy circles indi- 
cate females, that are supposed to carry the factors, but 





Fig. 112. — Nine banded Armadillo. Embryonic blastocyst that has 
four embryos on it, two of which are seen in figure. (After Newman and 
Patterson.) 


such females do not exhibit the character themselves. 
Solid black circles stand for affected females. 
Hemophilia appears in affected stocks almost ex- 
clusively in males (Fig. 113). Such males, mating 
with normal females, give only normal offspring, but 
the daughters of such unions if they marry normal 
males will transmit the disease to half of their sons. 
Affected females can arise only when a hemophilious 
male marries a female carrying hemophilia. If we 


HEREDITY AND SEX 


40) 


(}IOdueaeq WO.) “Wosso'T 0} Sulpioooe ‘Apres Jodureyy JO veiZpog = “elrydowyy yo AyIpossz]T — “ETT ‘pI a 


"er ae OOM OC) 7 O LI 





SPECIAL CASES OF SEX-INHERITANCE 241 


= SS XO UM 
a/ 
se <)> 


Q Ss 


XX XO 


SD BKK XX XO XO 


Fria. 114. — Diagram to indicate heredity of color blindness through 


male. <A color-blind male (here black) transmits his defect, to his grandsons 
only. 





e 


Fia. 115. — Diagram to indicate heredity of color blindness through 
female. <A color-blind female transmits color blindness to all of her sons, 
to half of her granddaughters and to half of her grandsons. 


242 HEREDITY AND SEX 


substitute white eyes for hemophilia, the scheme 
already given for white versus red eyes in flies applies 
to this case. If, for instance, the mother with normal 
eyes has two X chromosomes (Fig. 114), and the fac- 
tor for hemophilia is carried by the single X in the 
male (black X of diagram), the daughter will have 
one affected X (and in consequence will transmit the 
factor), but also one normal X which gives normal 


_ 


Fic. 116. — Pedigree of Ichthyosis from Bramwell. (After Davenport.) 


vision. The sons will all be normal, since they 
get the X chromosomes from their mother. In the 
next generation, as shown in the diagram (third line), 
four classes arise, normal females, hybrid females, normal 
males, and hzemophilious males. Color blindness fol- 
lows the same scheme, as the above diagrams illustrate 
(Figs. 114 and 115). In the first diagram the color- 
blind male is represented by a black eye; the normal 
female by an eye without color. The offspring from 


243 


SPECIAL CASES OF SEX-INHERITANCE 


(‘qaodu0aeg Joyfy) ‘“WIeYysuTIoFY Woy ‘ATIUIey 1008 yJ-Saveg ul Aydorye IBpNOsnuT JO voISIpIg — “LTT ‘Dy 
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244 HEREDITY AND SEX 


two such individuals are normal, but the color blindness 
reappears in one-fourth of the grandchildren, and in 
these only in the males. The reverse mating is shown 
in the next diagram in which the female is color-blind. 
She will have color-blind sons and normal daughters 
(criss-cross inheritance), and all four kinds of grand- 
children. 

Other cases in man that are said to show sex-linked 
inheritance are atrophy of the optic nerve, multiple 





Fic. 118. — Pedigree of night blindness in a negro family from Bordley. 
(After Davenport.) 


sclerosis, myopia, ichthyosis (Fig. 116), muscular 
atrophy (Fig. 117), and night-blindness (Fig. 118). 
There are also other cases in man that appear to come 
under the same category, but for which the evidence 
is not so clear. 

All these cases of sex-linked inheritance in man 
are explained by the assumption that the factor that 
produces these characters is carried by the sex chromo- 
some, which is duplex (XX) in the female and simplex 
(X) in the male. A simpler assumption has not yet 
been found. If one is fastidious and objects to the 


SPECIAL CASES OF SEX-INHERITANCE 245 


statement of factors being carried by chromosomes, he 
has only to say, that if the factors for the characters 
follow the known distribution of the sex chromosome, 
the results can be accounted for. 

The culmination of the evidence of sex-determina- 
tion in man is found in a study of the cell structure 
of the human race itself. Strange as it may seem, we 
have been longer in doubt concerning the number 
of chromosomes in man than in any other animal as 
extensively studied. Four conditions are responsible : 
(1) The large number of chromosomes present in man. 
(2) The clumping or sticking together of the chromo- 
somes. (3) The difficulty of obtaining fresh material. 
(4) The possibility that the negro race has half as many 
chromosomes as the white race. 

Two years ago Guyer announced the discovery that 
in all probability there exist in man two unpaired 
chromosomes in the male (Fig. 119) that behave in all 
respects like that in the typical cases of the sort in 
insects, where, as we have seen, there are two classes 
of spermatozoa, differing by the addition of one more 
chromosome in one class. ‘These produce females; the 
lacking class produces males. But Guyer’s evidence 
was not convincing. He found in all 12 chromosomes 
in one class of sperm and 10 in the other. Mont- 
gomery has also studied the same problem, but his 
account, while confirming the number, is in disagree- 
ment in regard to the accessory. 

Jordan has gone over a number of other mammals, 
in-some of which he thinks that he has found indica- 
tions at least of two classes of sperm. 

Still more recently another investigator, von Wini- 


246 HEREDITY AND SEX 


warter, has attacked the problem (Fig. 120). His 
material and his methods appear to have been superior 
to those of his predecessors. His results, while stated 
with caution and reserve, seem to put the whole 
question on a safer basis. 

His main results are illustrated in the diagram 





Fic. 119. — Human spermatogenesis according to Guyer. The sex 
chromosomes are seen in 6-9. 


(Fig. 120). In the male he finds 47 chromosomes. 
Of these 46 unite at reduction to give 23 double 
chromosomes — one remains without a mate. At the 
first reduction division the pairs separate, 23 going 
to each pole, the unpaired chromosome into one cell 
only. 


SPECIAL CASES OF SEX-INHERITANCE 247 


At the next division all the chromosomes in the 23 
eroup divide, likewise all in the 24 group divide. 
There are produced two spermatozoa containing 24 


j es 
5 Ome 
Ne va ‘ ee 
\ / YS) af. ‘We, 
\J a> aN A) | a #9 
N= Ke , / t 
j es*y ‘ } \ 
Bors | ” 
j st 
a / 
ag: 
bide 7 } 
ve Ne, ee 
-¢lk =k. Whit 
ee NEN , oe 
d of 
/; f 
: Ze whats 
’ q) © a. 
6, $ aie, wet 
tis Noghy ’ 
i gett 
fe ‘ (oN re 
ae ; ee 
; h aie 
t 
fa, 
Soe Ae ea, 
@)> Al ar % a ett 
Poy} LJ fixes ae +3 
“~ “9? ee t9° a2, 4 
, Aas a kee 
. ie, eae 
J 2 k f re 
Fia. 120. — Human spermatogenesis according to von Winiwarter. a, 


spermatogonial cell with duplex number; b, synapsis; c, d, e, f, first spermato- 
cytes with haploid number of chromosomes; g, first spermatocyte division, 
sex chromosomes (below) in advance of others; h, two polar plates of later 
stage; 7, first division completed; 7, second spermatocyte with 23 chromo- 
somes; k, second spermatocyte with 24 chromosomes; /, second spermato- 
cyte division; m, two polar plates of later stage. 


248 HEREDITY AND SEX 


chromosomes, and two containing 23 chromosomes ; 
all four sperms having come from the same spermato- 
gonial cell (Fig. 121). 

In the female von Winiwarter had difficulty in deter- 
mining the number of chromosomes present. His 


- 
Sex delermtination tn Man (, Mintwarcler J 


je Se ‘ : A® . iat Ge 
| o~ . ~ } = / a4 es ie A 
\ NAGE Ne . 24 
/ pe r , 2+ , 
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AT : nye? UAE ERO es 
A A Nat I oe es | 
234K if Sieh 
a eae g ase | 
micnurennees er 
jis, \ wy et Cr wy ) 
{ =e ay ey far 3 NZ, 
a . z / : hem. li Pinay, 23 23 
“RBtX- ‘ Woh : ‘onvmeenttoP™ 23 
’ ~\ . 5 ‘ j r/ pes ‘ 
B . \| / Af / yee f 
Y ie \ yee Foe) 
SIO 
o . el 
E 23 
F 
Fie. 121. — Diagram of human spermatogenesis. A, spermatogonial 


cell with 47 chromosomes; B, first spermatocyte with reduced haploid number 
and sex chromosome (open circle) ; C, first division; D, two resulting cells 
= second spermatocytes; FE, division of second spermatocytes; F, four 
resulting spermatozoa, two female-producing (above), two male-produc- 
ing (below). 


best counts gave 48 chromosomes for the full or duplex 
number. These observations fit in with the results 
from the male. 

If these observations are confirmed, they show that 
in man, as In so many other animals, an internal 
mechanism exists by which sex is determined. It is 
futile then to search for environmental changes that 


SPECIAL CASES OF SEX-INHERITANCE 249 


might determine sex. At best the environment may 
slightly disturb the regular working out of the two 
possible combinations that give male or female. Such 
disturbances may affect the sex ratio but have nothing 
to do with sex-determination. 





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BIBLIOGRAPHY 


ANDREws, E. A., 1895. Conjugation in an American Crayfish. 
Am. Nat., X XIX. 

ANDREWS, E. A., 1904. Breeding Habits of Crayfish. Am. Nat., 
XXXVIII. 

ANDREWS, E. A., 1910. Conjugation in the Crayfish Cambarus 
affnis. Jour. Exp. Zoél., IX. 

ARKELL, T. R., 1912. Some Data on the Inheritance of Horns in 
Sheep. JN. H. Agr. Exp. Sta. Bulletin, 160. 

ARKELL, T. R., and C. B. DAvENPorT, 1912. The Nature of the 
Inheritance of Horns in Sheep. Science, N.S., XX XV. 

ARKELL, T. R., and C. B. DAvENportT, 1912. Horns in Sheep as a 
Typical Sex-limited Character. Science, N. 8., XX XV. 

vON Barur, W. B., 1908. Ueber die Bildung der Sexualzellen bei d. 
Aphidide. Zool. Anz., XX XIII. 

von Barner, W. B., 1909. Die Oogenese bei einigen viviparen 
Aphiden und die Spermatogenese von Aphis saliceti. Arch. f. 
Zellf., 11. 

von Barur, W. B., 1912. Contribution a l'étude de la caryo- 
kinese somatique, de la pseudoréduction et de la réduction. 
La Cellule, XX VII. 

BarrsELu, G. A., 1912. Experiments on the Reproduction of the 
Hypotrichous Infusoria. Jour. Hxp. Zoél., XIII. 

Baurzmr, F., 1908. Uber die Grésse und Form der Chromosomen 
von Seeigeleiern. Verh. d. deutsch. zool. Ges., 1908. 

Baurzer, F., 1909. Die Chromosomen von Strongylocentrotus 
lividus und Echinus microtuberculatus. Arch. f. Zellf., II. 
Battzer, F., 1910. Ueber die Beziehung zwischen dem Chromatin 

und der Entwicklung und Vererbungsrichtung bei Echinoder- 
menbastarden. Arch. f. Zellf., V. 
Baurzer, F., 1911. Zur Kentnis der Mechanik der Kernteilungs- 
figuren. Arch. Entw.-mech. f. d. Org., XX XIII. 
Barrson, W., 1909. Mendel’s Principles of Heredity. Cam- 
bridge. 
201 


252 BIBLIOGRAPHY 


Bateson, W., 1912. Biological Fact and the Structure of Society. 
Cambridge. 

Bateson, W., 1913. Mendel’s Principles of Heredity. 3d im- 
pression. Cambridge. 

Bateson, W., and R. C. Punnett, 1905. Rep. Evol. Conm., II. 

Bateson, W., and R. C. Punnett, 1908. The Heredity of Sex. 
Science, X XVII. 

Bateson, W., and R. C. Punnert, 1911. On the Interrelations of 
Genetic Factors. Proc. Roy. Soc., LX XXIV. 

Batrson, W., and R. C. Punnett, 1911. The Inheritance of the 
Peculiar Pigmentation of the Silky Fowl. Jour. Gen., I. 

Batrson, W., and R. C. Punnett, 1911. On Gametic Series In- 
volving Reduplication of Certain Terms. Jour. Gen., I. 

BAUMGARTNER, W. J., 1912. Observations on the Gryllide — IV. 

Copulation. Kans. Univ. Sct. Bull., V. 

Baur, E., 1911. Ein Fall von Faktorenkoppelung bei Antir- 

rhinum majus. Verh. naturf. Ver. Brinn, XLIX. 

Baur, E., 1912. Vererbungs- und Bastardierungsversuche mit 
Antirrhinum—II. Faktorenkoppelung. Zeits. Abst. u._Ver- 
erb., VI. 

Baur, E., 1912. Ein Fall von geschlechtsbegrenzter Vererbung 
bei Melandrium album. Zeits. Abst. wu. Vererb., VIII. 

Brarp, J., 1902. The Determination of Sex in Animal Develop- 
ment. Zool. Jahrb. Abt. Anat., XVI. 

BEDDARD, F. E., 1906. Proc. Zodl. Soc. London, 1906, II, p. 533. 

vAN BrenepEN, Ep., and Cx. Junin, 1884. La spermatogenese 
chez l’ascaride megalocéphale. Bull. de Vacad. roy. de Bel- 
gique. Année 53, 3™° Ser., T. VII. 

Berry, E. H., 1906. The ‘‘ Accessory Chromosome” in Epeira. 
Biol. Bull., XI. 

BiackMAN, M.W., 1905. The Spermatogenesis of Myriapods. III. 
Bull. Mus. Comp. Zool., Harvard College, XLVIII. | 

BuAKESLEE, A. F., 1904. Sexual Reproduction in the Mucorinee. 
Proc. Amer. Acad. Arts and Sci., XL. 

BLAKESLEB, A. F., 1904. Zygospore Formation a Sexual Process. 
Science, XIX. 

BuAKESLER, A. F., 1906. Differentiation of Sex in Thallus Game- 
tophyte and Sporophyte. Bot. Gaz., XLII. 





BIBLIOGRAPHY 253 


BLAKESLEE, A. F., 1908. Sexual Condition in Fegatella. Bot. 
Gaz., XLVI. 

Borina, A. M., 1909. A Small Chromosome in Ascaris megalo- 
cephala. Arch. f. Zellf., IV. 

Boveri, Tu., 1907. Zellen Studien. Die Entwicklung dispermer 
Seeigel-Eier, ete. Jena. 

Boveri, Tu., 1908. Ueber die Beziehung des Chromatins zur 
Geschlechtbestimmung. Sitz. Phys.-Med. Gesell. Wiirzburg. 
Dez., 1908. 

Boveri, Tx., 1909. Die Blastomerenkerne von Ascaris megalo- 
cephala und die Theorie der Chromosomen-Individualitat. 
Arch. Zellf., III. 

Boveri, Tu., 1909. Uber ‘‘Geschlechtschromosomen” bei Ne- 
matoden. Arch. Zellf., IV. 

Boveri, TH., 1911. Ueber das Verhalten der Geschechtschro- 
mosomen bei Hermaphroditismus. Verh. Phys.-Med. Gesell. 
Wurzburg, XLI. 

Braver, A., 1893. Zur Kenntniss der Spermatogenese von Ascaris 
We alecetula: Arch. mkr. Anat., XLII. 

Bresca, G., 1910. Experimentelle i chaieeania ane die 
Pemidiren Sexualcharaktere der Tritonen. Arch. Entw.- 
mech., XXIX. 

Rrewster, E. T., 1906. Note on the Determination of Sex in 
Man. Am. Anthropol., VIII. 

Bripags, C. B., 1913. Partial Sex-Linkage in the Pigeon. Science, 
XXXVII. 

BucHner, Pau, 1911. Ueber hermaphrodite Seesterne. Zool. 
Anz., XX XVIII. 

Buanion, E., 1910. Les Cellules Sexuelles et la Détermination du 
Sexe. Bull. Soc. Vaudoise Sci. Nat., LXVI. 

Burcerr, H., 1913. Ueber Sexualitaét, Variabilitaét und Vererbung 
bei Phycomyces nitens. Ber. deutsch. Bot. Gesell., X. 

Cauxkins, G. N., 1902. Studies on the Life-History of Protozoa. 
Piper biol. pull Li: 

Cauxins, G. N., 1904. Studies on the Life-History of Protozoa. 
IV Jour. Hap. Zool., I. 

Cauxins, G.N.,1911. Protozoén Germ Plasm. Pop. Sci. Monthly. 

Caukins, G. N., 1913. Further Light on the Conjugation of Para- 
mecium. Proc. Soc. Exp. Biol. Med., X. 


254 BIBLIOGRAPHY 


Cains, G. N., and C. C. Lres, 1902. Studies on the Life-History 
of Protozoa, II. Arch. Protist., 1902. 

CARMICHAEL, I. 8., and F. H. A. MarsHauu, 1908. On the Oc- 
currence of Compensatory Hypertrophy in the Ovary. Jour. 
Physiol. XXXVI. 

CastLe, W. E., 1908. The Heredity of Sex. Bull. Mus. Comp. 
Zo06!., Harvard College, XL. 

CastLe, W. E., 1909. A Mendelian View of Sex Heredity. Science, 
XXIX. 

CastLE, W. E., 1910. The Effect of Selection upon Mendelian 
Characters Manifested in One Sex only. Jour. Exp. Zodl., 
VIEv . 

CastLE, W. E., 1911. Double-mating of Silkworm Moths. 
Science, XX XIII. 

CastLE, W. IX., 1911. Heredity in Relation to Evolution and 
Animal Breeding. New York. 

Caste, W. E., 1912. ' Are Horns in Sheep a Sex-limited Character ? 
Science, XXXV. 

Castie, W. I., 1913. Simplification of Mendelian Formule. Am. 
Nat., XLVII. 

Caste, W. E., F. W. CARPENTER, A. H. Cuark, 8. O. Mast, and 
W. M. Barrows, 1906. The Effects of Inbreeding, Cross- 
breeding, and Selection upon the Fertility and Variability of 
Drosophila. Proc. Am. Acad. Arts and Sct., XUI. 

Caste, W. E., and J. C. Paruuirs, 1909. <A Successful Ovarian 
Transplantation in the Guinea-pig, etc. Scrence, XXX. 

CurpestEr, F. E., 1911. The Mating Habits of Four Species of 
the Brachyura. Biol. Bull., XI. 

CuHoparD, L., 1912. Note sur un cas de gynandromorphisme chez 
Forficula auricularia. Arch. Zool. Exp. et Gen., Ser. 5, T. X. 

Cog, L. J., 1912. A Case of Sex-linked Inheritance in the Do- 
mestic Pigeon. Science, XXXVI. 

Couuins, G. N., 1912. Gametic Coupling as a Cause of Correla- 
tions. Am. Nat., XLVI. | 

Couton, H. 8., 1912. Lymnza columella and Self-fertilization. 
Proc. Acad. Nat. Scr. Phil. 

ConkLIN, E. G., 1906. Sex-differentiation in Dinophilus. Science, 
XXIV. 


BIBLIOGRAPHY 265 

Cook, M. H., 1910. Spermatogenesis in Lepidoptera. Proc. Acad. 
Nat. Scr. Phil. 

CorrENs, C., 1907. Zur Kenntnis der Geschlechtsformen poly- 
gamer Bliitenpflanzen und ihrer Beeinflussbarkeit. Jahrb. f. 
Wiss. Bot., XLIV. 

Correns, C., 1907. Die Bestimmung und Vererbung des Ge- 
schlechts. Berlin. 

CorRENS, C., 1908. Weitere Untersuchungen iiber die Geschlechts- 
formen polygamer Bliitenpflanzen und ihre Beeinflussbarkeit. 
Jahrb. f. wiss. Bot., XLV. 

Correns, C., 1908. Die Rolle der Mannlichen Keimzellen bei 
der Geschlechtsbestimmung der gynodioecischen Pflanzen. 
Ber. deutsch. Bot. Ges., X XVI. 

Correns, C., 1908. Weitere Untersuchungen ueber die Ge- 
schlechtsformen polygamer Bliitenpflanzen und ihre Beein- 
flussbarkeit. Jahrb. f. wiss. Bot., XLV. 

Correns, C., 1912. Selbststerilitét und Individualstoffe. Fests. 
Med. Nat. Gesell. 84, Versam. deut. Natur. Aerate. 

Correns, C., and R. GoupscumiptT, 1913. Die Vererbung und 
Bestimmung des Geschlechts.. Gesamtsitz. | naturw. mediz. 
Hauptg. 84. Vers. deuts. Naturf. u. Aerate in Minster. 

CuEnNotT, L., 1899. Sur la determination du sexe chez les animaux. 
Bull. Scient. France Belg., XXXII. 

CuNNINGHAM, J. T., 1908. The Heredity of Secondary Sexual 
Characters in Relation to Hormones. Arch. f. Entw.-mech., 
XX ViL. 

Darina, C. A., 1909. Sex in Dicecious Plants. Bull. Tor. Bot. 
Club, XXXVI. 

Darwin, C., 1886. The Descent of Man, and Selection in Relation 
to Sex. New York. 

Darwin, C., 1889. Different Forms of Flowers on Plants of the 
Same Species. New York. 

Darwin, C., 1895. Cross and Self-Fertilization in the Vegetable 
Kingdom. New York. 

Davenport, C. B., 1906. Inheritance in Poultry. Carnegie Inst. 
Wash., pub. 53. 

Davenport, C. B., 1909. Inheritance of Characteristics in Do- 
mestic Fowl. Carnegie Inst. Wash., pub. 121. 


256 BIBLIOGRAPHY 


Davenport, C. B., 1911. Heredity in Relation to Eugenics: New 
York. 

Davenport, C. B., 1911. Another Case of Sex-limited Heredity 
in Poultry. Proc. Soc. Exp. Biol. Med., IX. 

Davenport, C. B., 1912. Sex-limited Inheritance in Poultry. 
Jour. Exp. Zoél., XIII. 

Deperer, P. H., 1907. Spermatogenesis in Philosamia cynthia. 
Biol. Bull., XIII. 

DeaGcE, Y., 1908. Les Idées nouvelles sur la Parthénogénése 
expérimentale. La Revue des Idées, 50. 

De.court, A., and E. GuyEnot, 1911. Génétique et Milieu. Bull. 
Scient. France Belg., XLV. 

DELLA VALLE, P., 1913. Die Morphologie des Zellkerns und die 
Physik der Kolloide. Zeits. Chem. Industr. Kolloide, XII. 
Dexter, J. S., 1912. On Coupling of Certain Sex-linked Char- 

acters in Drosophila. Biol. Bull., XXIII. 

Dopps, G. 8., 1910. Segregation of the Germ-Cells of the Teleost, 
Lophius. Jour. Morph., X XI. 

DoncastER, L., 1907. Inheritance and Sex in Abraxas grossu- 
lariata. Nature, LX XVI. 

DoncastrErR, L., 1907. Gametogenesis and Fertilization in Nema- 
tus ribesi. Q. J. M.S., LI. 

Doncaster, L., 1908. On Sex-inheritance in the Moth, Abraxas 
grossulariata and its var. lacticolor. 4th Rep. Evol. Comm., 
R. Soc. Lond. 

Doncaster, L., 1908. Animal Parthenogenesis. Sci. Progr. 

Doncaster, L., 1909. Recent Work on the Determination of Sex. 
DCE TOG) YS LLL. 

Doncaster, L., 1910. Gametogenesis of the Gall-fly, Neuroterus 
lenticularis (Spathegaster baccarum). Part I. Proc. Roy. 
Soc. By L XXXL. 

Doncaster, L., 1911. Gametogenesis of the Gall-fly, Neuroterus 
lenticularis. Part II. Proc. Roy. Soc., LX XXIII. 

Doncaster, L., 1911. Note on the Inheritance of Characters in 
which Dominance appears to be Influenced by Sex. Jour. 
Genet., I. 

DoncastrrR, L., 1911. Some Stages in the Spermatogenesis of 
Abraxas grossulariata and its var. lacticolor. Jour. Genet., I. 


BIBLIOGRAPHY 257 


Doncaster, L., 1912. The Chromosomes in the Odgenesis and 
Spermatogenesis of Pieris brassicae, and in the Odgenesis of 
Abraxas grossulariata. Jour. Genet., Il. | 

DoncastTER, L., 1912. Note on the Chromosomes in Odgenesis 
and Spermatogenesis of the White Butterfly, Pieris brassice. 
Proc. Camb. Phil. Soc., XVI. 

Doncaster, L., and G. Gray, 1911. Cytological Observations on 
Cross-fertilized Kchinoderm Eggs. Proc. Camb. Phil. Soc., 
XVI. 

Doncaster, L., and F. H. A. MarsHatu, 1910. The Effects of 
One-sided Ovariotomy on the Sex of the Offspring. Jour. 
Genet., I. 

Doncaster, L., and G. H. Raynor, 1906. Breeding Experiments 
with Lepidoptera. Proc. Zoél. Soc. Lond. 

Drew, G. A., 1911. Sexual Activities of the Squid, Loligo Pealii 
(Hes.). I. Copulation, Egg-laying and Fertilization. Jour. 
Morph., XXII. 

DuruaM, F. M., 1911. Further Experiments on the Inheritance of 
Coat Colour in Mice. Jour. Genet., I. 

DuruaM, F. M., and D. E. C. Marryat, 1908. Note on the In- 
heritance of Sex in Canaries. 4th Rep. Evol. Comm., Roy. 
Soc. Lond. 

Kast, E. M., 1910. Notes on an Experiment concerning the Na- 
ture of Unit Characters. Science, XX XII. 

East, E. M., 1910. The Réle of Hybridization in Plant Breeding. 
Pop. Sci. Mon., Oct., 1910. 

Kast, E. M., 1911. The Genotype Hypothesis and Hybridization, 
Am. Nat., XLV. 

Kast, KE. M., 1912. A Study of Hybrids Between Nicotiana 
bigelovii and N. quadrivalvis. Bot. Gaz., LILI. 

Hast, E. M., and H. K. Hayss, 1911. Inheritance in Maize. 
Conn. Exp. Sta. Bull., 167. 

Hast, KE. M., and H. K. Hayss, 1912. Heterozygosis in Evolution 
and in Plant Breeding. U. S. Dept. Agric., Bureau Plant 
Ind. Bull. 243. 

Epwarps, ©. L., 1910. The Sex-determining Chromosomes in 
Ascaris. Science, XX XI. 

Emerson, R. A., 1909. Inheritance of Color in the Seeds of the 


258 BIBLIOGRAPHY 


Common Bean, Phaseolus vulgaris. Ann. Rep. Nebr. Agr. 
Exp. Sta., XXII. 

Emerson, R. A., 1911. Morgan on Coupling vs. Random Segrega- 
tion. Science, XXXIV. 

Emerson, R. A., 1911. Genetic Correlation and Spurious Alle- 
lomorphism in Maize. Ann. Rep. Nebr. Agr. Exp. Sia., 
XXIV. 

FrpERLEY, H., 1911. Vererbungsstudien an der Lepidopteren- 
Gattung Pygera. Arch. Rass. Gesell., I. 

Frre, C., 1898. Experiences relatives aux rapports homosexuels 
chez les hammetons. C. R. Soc. Biol., V. 

Frere, C., 1898. Experiences relatives a l’instinct sexuel chez le 
bombyx du murier. C. R. Soc. Biol., V. 

Fick, R., 1907. Uber die Vererbungssubstanz. Arch. Anat. wu. 
Phys., Anat. Abt. 

Foot, K., and E. C. Stroseui, 1913. Preliminary Note on the 
Results of Crossing Two Hemipterous Species, etc. Bizol. 
Bull., XXIV. 

Fow ter, G. H., 1894. Notes on some Specimens of Antlers of the 
Fallow Deer, ete. Proc. Zoél. Soc. Lond., 1894. 

Frotowa, 8., 1912. Idiochromosomen bei Ascaris megalocephala. 
Arch. Zellf., LX. 

Grppgs, P., and J. A. THomson, 1889. The Evolution of Sex. 
London. 

GEROULD, J. H., 1911. The Inheritance of Polymorphism and Sex 
in Colias philodice. Am. Nat., XLV. 

GoutpscHMipT, R., 1911. Einfiihrung in die Vererbungswissen- 
schaft. Leipzig. 

GoupscHMIpT, R., 1912. Erblichkeitsstudien an Schmetterlingen. 
I,1. Zetts. Abst. Vererb., VII. 

GoLpscHMIDT, R., 1912. Bemerkungen zur Vererbung des Ge- 
schlechtspolymorphismus. Zeits. Abst. Vererb., VIII. 

GoupscumipT, R., 1913. Zuchtversuche mit Enten. I. Zeits. 
Abst. Vererb., III. 

GoopaLEg, H. D., 1909. Sex and its Relation to the Barring Factor 
in Poultry. Scrence, X XIX. 

GoopaLE, H. D., 1910. Breeding Experiments in Poultry. Proc. 
Soc. Exp. Biol. Med., VII, 


BIBLIOGRAPHY 259 


GoopaLn, H. D., 1910. Some Results of Castration in Ducks. 
Biol. Bull., XX. 

GoopaLE, H. D., 1911. Studies on Hybrid Ducks. Jour. Exp. 
Zool., X. 

GoopaLE, H. D., 1911. Sex-limited Inheritance and Sexual Di- 
morphism in Poultry. Scvence, XX XIII. 
GoopaLg, H. D., 1913. Castration in Relation to the Secondary 
Sexual Characters of Brown Leghorns. Am. Nat., XLVII. 
Gregory, R. P., 1911. Experiments with Primula sinensis. Jour. 
Genet., I. 

Greoory, R. P., 1911. On Gametic Coupling and Repulsion in 
Primula sinensis. Proc. Roy. Soc., LXXXIV. 

Gross, J., 1912. Heterochromosomen und Geschlechtsbestimmung 
bei Insekten. Zodél. Jahrb., XXXII. 

GRUENBERG, B. C., 1912. Physiological Sex-determination. Science, 
Ne A XV: 

GUDERNATSCH, J. F., 1911. Hermaphroditismus verus in Man. 

Am. Jour. Anat., XI. 

GUDERNATSCH, J. F., 1912. Feeding Experiments on Tadpoles. I. 

Arch, Entw.-mech., XXXV. 

GUDERNATSCH, J. F., 1912. Fiitterungsversuche an Amphibien- 

larven. Zentrbl. f. Physiol., X XVI. 

Guuick, A., 1911. Ueber die Geschlechtschromosomen bei einigen 

Nematoden. Arch. f. Zellf., VI. 

Guymr, M. F., 1902. Hybridism and the Germ Cell. Univ. of 
Cincinnati Bull., No. 21. 

Guyer, M. F., 1903. The Germ Cell and the Results of Mendel. 
Cincinnati Lancet-Clinic. 

Guyer, M. F., 1905. Guinea-Chicken Hybrids. Science, N. S., 
XT: 

GuymrR, M. F., 1905. Notes on Cross-bred Chickens. Science, 
as ee @.@ Ri 

GuyER, M. F., 1907. Do Offspring Inherit Equally from Each 
Parent? Science, N.8., XXV. 

Guyer, M. F., 1909. Atavism in Guinea-Chicken Hybrids. Jour. 
Exp. Zool., VII. 

Guyer, M. F., 1909. La Livrée du Plumage chez les Hybrides 
de Pintade et de Poule. Bull. Mus. @hist. nat. 





260 BIBLIOGRAPHY 


Guyer, M. F., 1909. The Spermatogenesis of the Domestic 
Chicken. Anat. Anz., XXXIV. 

GuyrrR, M. F., 1909. The Spermatogenesis of the Domestic 
Guinea. Anat. Anz., XXXIV. 

Guysr, M. F., 1909. Deficiencies of the Chromosome Theory of 
Heredity. University Studies (Cincinnatr), Series II, Vol. V. 

Guysr, M. F., 1909. On the Sex of Hybrid Birds. Biol. Bull., 
XVI. 

Guyer, M. F., 1910. Accessory Chromosomes in Man. Biol. 
Bull., XTX. 

Guyer, M. F., 1911. Nucleus and Cytoplasm in Heredity. Am. 
Nat., XLV. 

Haptey, P. B., 1910. Sex-limited Inheritance: Science, XX XII. 

HaaceEpoorn, A. L., 1909. Mendelian Inheritance of Sex. Arch. f. 
Entw.-mech., XXVIII. 

Haaepoorn, A. L., 1912. The Genetic Factors in the Develop- 
ment of the House Mouse, ete. Zeits. Abst. Vererb., VI. 

Hart ey, C. P., E. B., Brown, C. H., Kyiz, and L. L. Zoox, 1912. 
Cross-breeding Corn. U.S. Dept. Agr. Bull., 218. 

Hartman, F. A., 1913. Variations in the Size of Chromosomes. 
Biol. Bull., XXIV. 

Haspmr, M., 1911. Zur Entwicklung der Geschlechtsorgane von 
Chironomus. Zool. Jahrb., Abt. Anat., XXXI. 

Heart, W., 1897. Further Note on the Transplantation and 
Growth of Mammalian Ova within a Uterine Foster-mother. 
Proc. Roy. Soc., LXII. 

Heapeg, W., 1899. Note on the Fertility of Different Breeds of 
Sheep. Proc. Roy. Soc., LXV. 

Hearg, W., 1905. Ovulation and Degeneration of Ova in the 
Rabbit. Proc. Hoy. Soc:, B, LXXVIk 

Hare, W., 1907. Notes on the Proportion of the Sexes in Dogs. 
Proc. Camb. Phil. Soc., XIV. 

Hare, W., 1907. Note on the Influence of Extraneous Forces 
upon the Proportion of the Sexes produced by Canaries. 
Proc. Camb. Phil. Soc., XIV. 

Hare, W., 1908. Note on Russo’s Attempt to show Differen- 
tiation of Sex in the Ovarian Ova of the Rabbit. Proc. Camb. 
PRLSSOCr ess 


BIBLIOGRAPHY 261 


Hearst, W., 1908. The Proportions of the Sexes produced by 
Whites and Colored Peoples in Cuba. Phil. Trans. Roy. 
Soc. Lond., Series B, Vol. 200. 

Heiper, K., 1906. Vererbung und Chromosomen. Jena. 

HeENKING, H., 1891. Untersuchungen ueber die ersten Entwick- 
lungsvorgiinge in den Eiern der Insekten. Zevts. f. wiss. Zool., LI. 

Hersst, C., 1909. Vererbungstudien VI. Arch. Entw.-mech., 
XXVII. 

Hertwia, O., 1890. Vergleich der Ei und Samenbildung bei 
Nematoden. Arch. mikr. Anat., XXXVI. 

Hertwia, R., 1906. Ueber Knospung und Geschlechtsentwicke- 
lung von Hydra fusca. Festschr. f. J. Rosenthal. Leipzig. 

Hertwic, R., 1907. Uber den Chromidialapparat und den 
Dualismus der Kernsubstanzen. Sitz. ber. Gesell. Morph. und 
Phys. in Miinchen. 1907. 

Hertwie, R., 1907. Untersuchungen iiber das Sexualitaétsproblem. 
Ill. Teil. Verh. Deutsch. Zool. Gesell. 

Hertwia, R., 1912. Uber den derzeitigen Stand des Sexualitiits- 
problems nebst eigenen Untersuchungen. vol. Centr., 
XXXII. 

Ho.pina, R. E., 1905. Exhibition of, and Remarks upon, a Series 
of the First Year Antlers of Certain Deer. Proc. Zodél. Soc. 
Lond., II. 

Hotmgss, 8. J., 1903. Sex Recognition among Amphipods. Bzvol. 
Beas 

JANDA, V., 1912. Die Regeneration der Geschlechtsorgane bei 
Criodrilus lacuum. Arch. Entw.-mech., XXXIV. 

JANNSENS, F. A., 1901. La spermatogénése chez les tritons. La 
Cellule, XIX. 

JANNSENS, F. A., 1905. Evolution des Auxocytes males du Batra- 
coseps attenuatus. La Cellule, XXII. 

JANNSENS, F. A., 1909. La théorie de la chiasmatypie. Nou- 
velle interprétation des cinéses de maturation. La Cellule, 
XXV. 

JANNSENS, F. A., and R. Dumez, 1903. L’élément nucléinien 
pendant les cinéses de maturation des spermatocytes chez 
Batracoseps attenuatus et Plethedon cinereus. La Cellule, 


XX. 


262 BIBLIOGRAPHY 


JENNINGS, H. S., 1908. Heredity, Variation and Evolution in 
Protozoa, II. Proc. Am. Phil. Soc., XLVII. , 

JENNINGS, H. S., 1909. Heredity and Variation in the Simplest 
Organisms. Amer. Nat., XLIII. 

JENNINGS, H. S., 1910. Experimental Evidence of the Effective- 
ness of Selection. Amer. Nat., XLIV. 

JENNINGS, H. S8., 1911. Assortative Mating, Variability and In- 
heritance of Size in the Conjugation of Paramoecium. Jour. 
Exp. Zoél., XI. 

JENNINGS, H. §., 1911. Pure Lines in the Study of Genetics in 
Lower Organisms. Amer. Nat., XLV. 

JENNINGS, H.8.,1913. The Effect of Conjugation in Paramecium. 
Jour. Exp. Zoél., XIV. 

JENNINGS, H.8., and G. T. Hararrt, 1910. Characteristics of the 
Diverse Races of Paramecium. Jour. Morph., XI. 

JENNINGS, H.S., and K. 8. LAsHiey, 1913. Biparental Inheritance 
and the Question of Sexuality in Paramecium. Jour. Exp. 
Zool., XIV. 

JOHANNSEN, W., 1909. Elemente der exakten Erblichkeitslehre. 
Jena. 

JOHANNSEN, W., 1911. The Genotype Conception of Heredity. 
Am. Nat., XLV. 

Jones, W. N., 1912. Species Hybrids of Digitalis. Jowr. Genet., 
Il. 

Kaun_e, W., 1908. Die Pedogenesis der Cecidomyiden. Stuttgart. 

KEEBLE, F., and C. PELLEw, 1910. The Mode of Inheritance of 
Stature and of Time of Flowering in Peas (Piswm sativum). 
Jour. Genet., I. 

Kewioaa, V. L., 1907. Some Silkworm Moth Reflexes. Biol. 
Bull Xi 

Kerxuioae, V. L., 1907. Artificial Parthenogenesis in the Silkworm. 
Biol. Bull., XIV. 

Kewuoaa, V. L., 1911. An Experiment in Double Mating. Science, 
XXXIII. 

Kina, H. D., 1907. Food as a Factor in the Determination of Sex 
in Amphibians. Biol. Bull., XIII. 

Kina, H. D., 1909. Studies on Sex-Determination in Amphibians. 
II. Biol. Bull., XVI. 


BIBLIOGRAPHY 263 


Kine, H. D., 1910. Temperature as a Factor in the Determination 
of Sexin Amphibians. vol. Bull., XVIII. 

Kine, H. D., 1910. Some Anomalies in the Genital Organs of 
Bufo lentiginosus and their Probable Significance. Am. 
Jour. Anat., X: 

Kina, H. D., 1911. Studies on Sex-Determination in Amphibians. 
DV els Bill; XX 

Kine, H. D., 1911. The Sex Ratio in Hybrid Rats. Biol. Bull., 
XXII. 

Kine, H. D., 1912. Dimorphism in the Spermatozoa of Necturus 
maculosus. Anat. Rec., VI: 

Kina, H. D., 1912. Studies on Sex-Determination in Amphibians. 
Meee ourelrp- 7001..-x 11, 

Kopsc, 8., 1911. Untersuchungen iiber Kastration und Trans- 
plantation bei Schmetterlingen. Arch. Entw.-mech., XX XIII. 

Kruacer, Eva, 1912. Die phylogenetische Entwicklung der 
Keimzellenbildung einer freilebenden Rhabditis. Zool. Anz., 
2.2 Fp 

KuscHAKEwitTscH, 8., 1910. Die Entwickelungsgeschichte der 
Keimdriisen von Rana esculenta. Festschr. f. R. Hertwig, 
Bd. II, 1910. Jena. 

KuscHAkeEwitTscuH, 8., 1911. Ein Fall von Hermaphroditismus 
lateralis verus bei Rana esculenta. Anat. Anz., XX XVIII. 
KUSCHAKEWITSCH. S8., 1911. Erklarung zur Notiz von T. H. 
Morgan: “Is the female frog heterozygous in regard to sex- 

determination?” Anat. Anz., XX XIX. 

Lane, A., 1912. Vererbungswissenschaftliche Miszellen. Zetts. 
Abst. Vererb., VIII. 

von LenHossé&kK, 1903. Das Problem der geschlechtsbestimmenden 
Ursachen. Jena, 1903. 

Litre, C. C., 1912. Preliminary Note on the Occurrence of a Sex- 
limited Character in Cats. Science, XXXYV. 

Logs, J., 1908. Hervorrufung der Membranbildung, ete. Arch. 
Gesell. Phys., CX_XII. 

Lors, J., 1908. Entwicklungserregung unbefruchtete Anneli- 
deneier, etc. Arch. Gesell. Phys., CX XII. 

Logs, J., 1909. Die Chemische Entwicklungserregung des 
tierischen Hies. Berlin. 


264 BIBLIOGRAPHY 


Lors, J., 1911. The Réle of Salts in the Preservation of Life. 
Science, XX XIV. 

Logs, J., 1912. Heredity in Heterogeneous Hybrids. Jour. 
Morph., XXIII. 

Lors, J.. and F. W. Bancrort, 1911. Some Experiments 
on the Production of Mutants in Drosophila. Science, 
XXXII. 

Lorp, J., and F. W. Bancrort, 1913.. The Sex of a Parthenogenetic 
Tadpole and Frog. Jour. Exp. Zo6l., XIV. 

Lors, J., und H., Wasreneys, 1911. Die Beeinfliissung der 
Entwicklung, etc. Biroch. Zeits., XX XVII. 

Logs, J.. und H. Wastenrys, 1911. Weitere Bemerkungen iiber 
den Zusammenhang zwischen Oxydationsgrésse und Cytolyse 
der Seeigeleier. Bioch. Zeits., XX XI. 

Lutz, A. M., 1912. Triploid Mutants in Oenothera. Bvrol. Centr., 
XXXII. 

Lutz, F. E., 1911. Experiments with Drosophila ampelophila 

concerning Evolution. Carnegie Inst. Wash., pub. 143. 

Lutz, F. E., 1913. Experiments concerning the Sexual Difference 
in the Wing Length of Drosophila ampelophila. Jowr. Exp. 
Zool., XIV. 

McC.uenpon, J. F., 1910. On the Effect of External Conditions 
on the Reproduction of Daphnia. Am. Nat., XLIV. 

McCuune, C. E., 1902. The Accessory Chromosome — Sex De-- 
terminant? Bvrol. Bull., U1. 

McCuuna, C. E., 1902. Notes on the Accessory Chromosome. 
Anat. Anz., XX. 

McCuuwna, C. E., 1905. The Chromosome Complex of Orthopteran 
Spermatocytes. Biol. Bull., UX. 

McCook, H. C., 1898. American Spiders and their Spinning 

Work. 3 vols. Philadelphia. 

McCrackeEn, I., 1909. Heredity of the Race-characters. Univol- 

tinism, etc.,in the Silkworm. Jowr. Exp. Zoél., VII. 

Mark, E. L., and U. Copr.anp, 1907. Maturation Stages in the 
Spermatogenesis of Vespa maculata. Proc. Am. Acad. Arts 
and Scv., XLII. 

MarsHauu, F. H. A., 1908. The Effects of Environment and 
Nutrition upon Fertility. Scv. Progr., VII. 





BIBLIOGRAPHY 2695 


MArsHALL, F. H. A., 1912. On the Effects of Castration and 

Ovariotomy on Sheep. Proc. Roy. Soc., LXXXV. 

MarsHa.., F. H. A., and W. R. Pevy, 1910. ‘‘Fatness’’ as a Cause 

of Sterility. Jour. Agri. Scv., III. 

Mast, 8. O., 1912. Behaviour of Fire-flies (Photinus pyralis ?) 
with Special Reference to the Problem of Orientation. Jour. 
An. Behav., I. 

Mayer, A. G., 1900. On the Mating Instinct in Moths. Ann. 
Mag. Nat. Hist., V. 

Mayer, A. G., and C. G. Souns, 1906. Some Reactions of Cater- 
pillars and Moths. Jour. Exp. Zoél., U1. 

DE Mrvers, J. C. H., 1910. Ueber Jacobsons Ziichtungsversuche 
beziiglich des Polymorphismus von Papilio Memnon. Zeits. 
Abst. Vererb., III. 

DE Mryere, J. C. H., 1910. Ueber getrennte Vererbung der 
Geschlechter. Biol. Centr., XXX. 

pe Meurer, J. C. H., 1911. Uber getrennte Vererbung der Ge- 
schlechter. Arch. Rass. Gesell., VIII. 

MertsENHEIMER, J., 1908. Uber den Zusammenhang von Ge- 
schlechtsdriisen und sekundaren Geschlechtsmerkmalen bei den 
Arthropoden. Verh. d. deutsch. Zool. Gesell., 1908. 

MEISENHEIMER, J., 1909. Experimentelle Studien zur Soma- und 
Geschlechtsdifferenzierung, I. Jena, 1909. 

MEISENHEIMER, J., 1912. Experimentelle Studien zur Soma und 
Geschlechtsdifferenzierung. Fests. zum 60 Geburtstage von 
Dr. J.W.Spengel. III. 

MENDEL, G., 1865. Versuche tiber Pflanzen-Hybriden. . Verh. 
d. Naturf. Vererns in Britnn, IV. 

Meves, Fr., 1903. Richtungskorperbildung in der Spermato- 
genese. Mittherl. f. d. Verein Schles.-Holst. Aerzte, XI. 

MeEvEs, F., 1907. Die Spermatocytenteilungen bei der Honigbiene, 
etc. Arch. mikr. Anat., LXX. 

Meves, Fr., and J. Dursspera, 1908. Die Spermatocytenteilung 
bei der Hornisse. Arch. mikr. Anat., LX XI. 

Meyns, R., 1912. Transplantation embryonaler und jugend- 
licher Keimdriisen u. s. w. Arch. mikr. Anat., LX XIX. 

Mogenxkuaus, W.J., 1910. Cross Fertilization among Fishes. Proc. 

Ind. Acad. Scr. 





266 BIBLIOGRAPHY 


MoenxuHaus, W. J., 1911. Effects of Inbreeding, etc., in Dro- 
sophila ampelophila. Jour. Morph., XXII. 

Montcomery, T. H., 1903. Studies on the Habits of Spiders, 
Particularly those of the Mating Period. Proc. Acad. Nat. Sci. 
Phila. 

Monrtcomemry, T. H., Jr., 1905. The Spermatogenesis of Syrbula 
and Lycosa, with General Considerations upon Chromosome 
Reduction, ete. Proc. Acad. Nat. Sci. Phil., 1905. 

Montaomery, T. H., Jr., 1906. Chromosomes in the Spermato- 
genesis of Hemiptera and Heteroptera. Trans. Am. Phil. 
RY iha 2. B 

Monteomery, T. H., Jr., 1906. The Terminology of Aberrant 
Chromosomes and their Behavior in certain Hemiptera. 
Science, XXIII. 

Montaomery, T. H., Jr., 1906. On Reproduction, Animal Life 
Cycles, and the Biological Unit. Trans. Tex. Acad. Sci., 
Ex : 

Montaomnmry, T. H., Jr., 1907. On Parthenogenesis in Spiders. 
Biol: Bull., XIII. 

Montaomery, T. H., Jr., 1908. The Sex-ratio and Cocooning Habits 
of an Aranead and the Genesis of Sex-ratios. Jour. Exp. 
Zool., V. 

Montaomery, T. H., Jr., 1910. The Cellular Basis of the Deter- 
mination of Sex. International Clinics, Vol. I. 

Montaomery, T. H., 1910. The Significance of the Courtship and 
Secondary Sexual Characters of Araneads. Am. Nat., XXXIV. 

Monrtcomery, T. H., 1910.. Further Studies on the Activities of 
Araneads. II. Proc. Acad. Nat. Sci. Phila., 1909. 

Montcommry, T. H., 1912. Human Spermatogenesis. Jour. 

Acad. Nat. Sci. Phila., XV. 

Moraan, T. H., 1903. Recent Theories in Regard to the De- 

termination of Sex. Pop. Sci. Mon., 1903. 

Moraan, T. H., 1905. Ziegler’s Theory of Sex-determination and 
an Alternative Point of View. Science, XXII. 

_ Moraan, T. H., 1905. An Alternative Interpretation of the Origin 
of Gynandromorphous Insects. Science, XXI. 

Moraan, T. H., 1906. The Male and Female Eggs of Phylloxerans 
of the Hickories. Bvrol. Bull., X. 





BIBLIOGRAPHY 267 


Moraan, T. H., 1907. Experimental Zodlogy. New York. 

Moraan, T. H., 1907. The Cause of Gynandromorphism in Insects. 
Am. Nat., XLI. 

Moraan, T. H., 1907. The Biological Significance and Control 
of Sex. Science, XXV. 

Moraan, T. H., 1908. The Production of two kinds of Sper- 

matozoa in Phylloxerans. Proc. Soc. Exp. Biol. and Med., V. 

Moraan, T. H., 1909. Sex-Determination and Parthenogenesis in 

Phylloxerans and Aphids. Science, XXIX. 

Morgan, T. H., 1909. Are the Drone Eggs of the Honey Bee 
Fertilized? Am. Nat., XLIITI. : 

Morean, T. H., 1909. Hybridology and Gynandromorphism. 
Am. Nat., XLIII. 

Moraan, T. H., 1909. A Biological and Cytological Study of 
Sex-Determination in Phylloxerans and Aphids. Jour. Exp. 
Zool., VII. : 

Moraan, T. H., 1910. Sex-limited Inheritance in Drosophila. 

Science, XXXII. 

Morean, T. H., 1910. Hybridization in a Mutating Period in 

Drosophila. Proc. Soc. Exp. Biol. and Med., VII. 

Moraan, T. H., 1910. The Chromosomes in the Parthenogenetic 
and Sexual Eggs of Phylloxerans and Aphids. Proc. Soc. 
Exp. Biol. and Med., VII. 

Morean, T. H., 1910. Chromosomes and Heredity. Am. Nat., 
XLIV. 

Morgan, T. H., 1910. The Method of Inheritance of two Sex- 

- limited Characters in the Same Animal. Proc. Soc. Exp. 
Biol. Med., VIII. 

Moraan, T. H., 1911. An Attempt to analyze the Constitution 
of the Chromosomes on the Basis of Sex-limited Inheritance 
in Drosophila. Jour. Exp. Zoél., XI. 

Moraan, T. H., 1911. The Application of the Conception of 
Pure Lines to Sex-limited Inheritance and to Sexual Dimorph- 
im. Am. Nat., XLV. 

Moraan, T. H., 1911. A Dominant Sex-limited Character. Proc. 
Soc. Exp. Biol. Med., IX. 

Moraan, T. H., 1911. An Alteration of the Sex-ratio induced by 
Hybridization. Proc. Soc. Exp. Biol. and Med., VIII. 








268 BIBLIOGRAPHY 


Moraan, T. H., 1911. Is the Female Frog Heterozygous in regard 
to Sex-determination? Am. Nat., XLV. 

Moraan, T. H., 1911. Random Segregation vs. Coupling in Men- 
delian Inheritance. Science, XXXIV. 

Moraan, T. H., 1911. Chromosomes and Associative Inheritance. 
Science, XX XIV. 

Moraan. T. H., 1911. The Origin of Nine Wmg-mutations in 
Drosophila. Science, XX XIII. 

Moraan, T. H., 1911. The Origin of Five Mutations in Eye-color 
in Drosophila. Sczence, XX XIII. 

Moraan, T. H., 1912. The Elimination of the Sex-chromo- 
somes from the Male-producing Eggs of Phylloxerans. Jour. 
Exp. Zool., XII. 

Morgan, T. H., 1912. Heredity of Body Color in Drosophila. 
Jour. Exp. Zoél., XIII. 

Moraan, T. H., 1912. A Modification of the Sex-ratio, and of 
other Ratios in Drosophila through Linkage. Zeits. Abst. 
Vererb., VII. 

Morean, T. H., 1912. Is the change in Sex-ratio of the Frog that 
is affected by External Agents, due to Partial Fertilization ? 
Am. Nat., XLVI. | 

Morean, T. H., 1912. The Explanation of a New Sex Ratio in 
Drosophila. Science, XXXVI. 

Moraan, T. H., 1912. Complete Linkage in the Second Chromo- 
some of the Male. Scirence, X XVI. 

Moraan, T. H., 1912. Further Experiments with Mutations in 
Kye-color in Drosophila. Jowr. Acad. Nat. Sct. Phil., XV. 
Morean, T. H., 1912. Eight Factors that show Sex-linked In- 

heritance in Drosophila. Science, XXXYV. 

Moraan, T. H., 1912. The Masking of a Mendelian Result by the 
Influence of the Environment. Proc. Soc. Exp. Biol. Med., IX. 

Moraan, T. H., 1913. Factors and Unit Characters in Mendelian 

Heredity. Am. Nat., XLVII. 

Morean, T. H., and E. Carrey, 1912. Data for the Study of Sex- 

linked Inheritance in Drosophila. Jour. Exp. Zoél., XIII. 

Morean, T. H., and E. Carrexy, 1913. Additional Data for the 
Study of Sex-linked Inheritance in Drosophila. Jour. Exp. 
Zool., XIV. 





BIBLIOGRAPHY 269 


Moraan, T. H., and H. D. Goopa.g, 1912. Sex-linked Inheritance 
meeoultry.. Ann NV. Y. Acad: scu.;, 3X11: 

Morean, T. H., and C. J. Lynceu, 1912. The Linkage of Two 
Factors in Drosophila that are not Sex-linked. Biol. Bull., 
XXIII. 

Morgan, T. H., F. Paynz, and E. N. Browns, 1910. A Method 
to test the Hypothesis of Selective Fertilization. Biol. Bull., 
XVIII. 

Morritu, C. V., 1909. Preliminary note on the Chromosomes in 
the Odgenesis, Fertilization and Cleavage of certain Hemiptera. 
Science, XXX. 

Morritt, C. V., 1910. The Chromosomes in the Odgenesis, Fertil- 

ization and Cleavage of Coreid Hemiptera. Biol. Bull., XTX. 

Morsgz, M., 1910. Sterility. Am. Nat., XLIV. 

Mutsow, K., 1912. Der Chromosomencyclus bei Ancyracanthus 

cystidicola Rud. Arch. f. Zellf., [X.°1. 

NacutsHEImM, H., 1912. Parthenogenese, Eireifung und Ge- 
schlechtsbestimmung bei der Honigbiene. Sitzwngsber. Gesell. 
Morph. u. Phys. Minchen. 

Newman, H. H., 1907. Spawning Behavior and Sexual Dimorph- 

ism in Fundulus heteroclitus and Allied Fish. Biol. Bull., XII. 

Newman, H. H., 1908. A Significant Case of Hermaphroditism in 

Fish. Biol. Bull., XV. 

Newman, H. H., and J. T. Parrerson, 1909. A Case of Normal 
Identical Quadruplets in the Nine-banded Armadillo. Bvol. 
Bull., XVII. 

Newman, H. H., and J. T. Parrerson, 1910. The Development of 
the Nine-banded Armadillo, ete. Jour. Morph., X XI. 

Newmav, H. H., and J.T. Parrerson, 1911. The Limits of Hered- 

ity Control in Armadillo Quadruplets. Jour. Morph., XXII. 

Nusspaum, M., 1909. Uber Geschlechtsbildung bei Polypen. 

Arch. Gesell. Physiol., CX XX. 

NutTA.., G. H., and G. Merriam, 1911. The Process of Copula- 
tion in Ornithodorus moubata. Parasitology, IV. 

Payne, F., 1912. A Further Study of the Chromosomes of the 
Reduviide, ete. Jour. Morph., XX XIII. 

Payne, F., 1912. The Chromosomes of Gryllotalpa borealis. 
Arch. f. Zellf.; TX, | 











270 BIBLIOGRAPHY 


Pear, M. and R., 1908. On the Relation of Race Crossing to the 
Sex Ratio. Biol. Bull., XV. 

Peary, R., 1911. Inheritance of Fecundity in the Domestic Fowl. 
Am. Nat., XLV. 

Pear, R., 1911. Breeding Poultry for Egg Production. Ann. 
Rep. Me. Agr. Exp. St., 1911. 

PrEarRL, R., 1912. The Mode of Inheritance of Fecundity in the 
Domestic Fowl. Jour. Exp. Zoél., XIII. 

PEARL, R., 1912. Notes on the History of Barred Breeds of 
Poultry. Biol. Bull., XXII. 

Pear, R., 1912. The Mendelian Inheritance of Fecundity in the 
Domestic Fowl. Am. Nat., XLVI. 

PEARL, R., and M. R. Curtis, 1912. Studies on the Physiology of 
Reproduction in the Domestic Fowl. V. Jour. Exp. Zodl., 
XIII. 

PrARL, R., and H. M. Parsuuey, 1913. Data on Sex Determination 
in Cattle. Biol. Bull., XXIV. 

PEARL, R., and F. M. Surrace, 1910. On the Inheritance of the 
Barred Color Pattern in Poultry. Arch. Entw.-mech., XXX. 

PEARL, R., and F. M. Surracsn, 1910. Studies on Hybrid Poultry. 
Papers Biol. Lab. Me. Agr. Exp. Sta., 21. 

PEARL, R., and F. M. Surracsg, 1910. Experiments in Breeding 
Sweet Corn. Papers Biol. Lab. Me. Agr. Exp. Sta., 18. 

PwrARL, R., and F. M. Surrace, 1910. Further Data Regarding the 
Sex-limited Inheritance of the Barred Color Pattern in Poultry. 
Science, XXXIT. 

PEARL, R., and F. M. Surrace, 1911. A Biometrical Study of Egg 
Production in the Domestic Fowl. U. S. Dept. Agric., Bur. 
Anim. Ind., Bull. 110. 

Prarss, A. 8., 1909. Observations on Copulation among Craw- 
fishes with Special Reference to Sex Recognition. Am. Nat., 
XLII. 

PrckHaM, G. W. and E. G., 1889. Observations on Sexual Selection 
in Spiders of the family Attide. Nat. Hist. Soc. Wis. Occa- 
sional Papers, I. 

PecxuaM, G. W. and E. G., 1890. Additional Observations, with 
some Remarks on Mr. Wallace’s Theory of Sexual Ornamenta- 
tion. Nat. Hist. Soc. Wis. Occasional Papers, I. 


BIBLIOGRAPHY 271 


PERGANDE, T., 1904. North American Phylloxerine affecting 
Hicoria and other Trees. Proc. Dav. Acad. Sci., IX. 

PETRUNKEWITscH, A., 1902. Das Schicksal der Richtungskorper 
im Drohnenei. Jena, 1902. 

PETRUNKEWwITSCH, A., 1910. Courtship in Dysdera crocata. Biol. 
Bull., X1X. 

Puruures, E. F., 1903. A Review of Parthenogenesis. Proc. Am. 
Ei s06., ULI. 

Puate, L., 1910. Die Erbformeln der Aglia tau-Rassen im An- 
schluss an die Standfusschen Ziichtungen. Arch. Rass. Gesell., 
VIL. 

Puate, L., 1913. Vererbungslehre. Leipzig. 

Pocock, R. I., 1905. The Effects of Castration on the Horns of 
a Prong Buck. Proc. Zoél. Soc. Lond., I. 

PortsR, J. P., 1906. The Habits, Instincts, and Mental Powers of 
Spiders, Genera Argiope and Epeira. Am. Jour. Psych., XVII. 

Ports, F. A., 1910. Notes on Free-Living Nematodes. I. 
Ore iV nS. iV. 

Punnetr?, R. C., 1903. On Nutrition and Sex-determination in 
Man. Proc. Cambr. Phil. Soc., XII. 

Punnett, R. C., 1906. Sex-determination in Hydatina, with 
some Remarks on Parthenogenesis. Proc. Roy. Soc., LX XVIII. 

PuUNNETT, R. C., 1909. On the Alleged Influence of Lecithin upon 
the Determination of Sex in Rabbits. Proc. Camb. Phil. Soc., 
UN: 

Punnett, R. C., 1910. ‘‘Mimicry” in Ceylon Butterflies, etc. 
Spol. Zeyl., VII. — 

Punnett, R. C., 1911. Mendelism. 

PunNnETT, R. C., 1912. Inheritance of Coat Color in Rabbits. 
Jour. Genet., IT. 

Rawts, E., 1913. Sex-ratios in Drosophila ampelophila. Bvol. 
Bull., XXIV. 

ReGen, J., 1909. Kastration und ihre Folgeerscheinungen bel 
Gryllus campestris. I. Zool. Anz., XXXIV. 

ReGen, J., 1910. Kastration und ihre Folgeerscheinungen bei 
Gryllus campestris. II. Zool. Anz., XXXV. 

Rippue, O., 1909. Our Knowledge of Melanin Color Formation 
and its Bearing on the Mendelian Description of Heredity. 
Biol. Bull., XVI. 


272 BIBLIOGRAPHY 


Rippie, O., 1912. Preliminary Chemical Studies on Male and 
Female Producing Eggs of Pigeons. Science, XX XV. 

Rippig, O., and A. A. Spon, 1912. On a Relation found to exist 
between Changes in the Chemical Composition of a Membrane 
and Changes in its Permeability. Science, XXXYV. 

SAUNDERS, E. R., 1911. Further Experiments on the Inheritance of 
‘““Doubleness”’ and Other Characters in Stocks. Jour. Genet., I. 

SaunpERS, E. R., 1911. Studies in the Inheritance of Doubleness 
in Flowers. I. Petunia. Jour. Genet., I. 

ScHierp, W., 1911. Variation, Hereditét, Bastardierung, De- 
scendenzlehre. Jahrb. u. d. For. d. Anat. u. Entwtschr. 
Gesch., XVI. 

ScHueip, W., 1912. Geschlechtsbestimmende Ursachen im Tier- 
reich. Hrgeb. und Fortschr. der Zool., U1. 

ScHuLTzE, O., 1902. Was lehren uns Beobachtung und Experi- ° 
ment itiber die Ursachen mannlichen und weiblicher Ge- 
schlechtsbildung, etc. ? Svtz.-ber. der Phys.-med. Gesell. Witrz- 
burg. 

ScuuttzE, O., 1903. Zur Frage von den geschlechtsbildenden 
Ursachen. Arch. mikr. Anat., LXIII. 

SEILER, J., 1913. Das Verhaltung der Geschlechtschromosomen 
bei Lepidopteren. Zool. Anz., XLI. 

SELIGMANN, C. G., 1906. Proc. Zool. Soc. Lond., II., p. 908. 

SHEARER, C., 1911. The Problem of Sex-Determination in Di- 
nophilus gyrociliatus. Jowr. Marine Biol. Ass., IX. 

SHEARER, C., 1912. The Problem of Sex-Determination in Di- 
nophilus gyrociliatus. Q. J. M.S., LVII. 

SHEARER, C., and D. J. Luoyp, 1913. On Methods of Producing 
Artificial Parthenogenesis in Echinus’ esculentus, ete. 
Qa Je Migs aly Le 

SHEARER, C., W. pE Moraan, and H. M. Fucus, 1911. Prelimi- 
nary Notice on the Experimental Hybridization of Echinoids. 
Jour. Marine Biol. Ass., IX. 

SHEARER, C., W. DE Morgan, and H. M. Fucus, 1912. On 
Paternal Characters in Echinoid Hybrids. Q.J.M.S., LVIII. 

SHULL, A. F., 1910. Studies in the Life Cycle of Hydatina senta. 
Jour. Exp. Zo6l., VAT. 

SHuuu, A. F., 1910. The Artificial Production of the Partheno- 


BIBLIOGRAPHY 273 


genetic and Sexual Phases of the Life Cycle of Hydatina senta. 
Am. Nat., XLIV. 

SHuuu, A. F., 1911. Studies in the Life Cycle of Hydatina senta. 
II. Jour. Exp. Zodl., X. 

SHULL, A. F., 1912. Studies in the Life Cycle of Hydatina senta. 
at. Jour. Exp. Zool., XII. 

SHuui, A. F., 1912. The Influence of Inbreeding on Vigor in Hy- 
datina senta. Biol. Bull., XXIV. 

SHULL, G. H., 1908. The Composition of a Field of Maize. Am. 
Breeders’ Assoc., IV. 

SHuLL, G. H., 1909. The ‘‘Presence and Absence” Hypothesis. 
Am. Nat., XLIII. 

SHuLL, G. H., 1909. A Pure Line Method in Corn Breeding. 
Am. Breed. Ass., V. 

SHULL, G. H., 1910. Hybridization Methods in Corn Breeding. 

Am. Breeders’ Mag., I. 

SHULL, G. H., 1910. Inheritance of Sex in Lychnis. Bot. Gaz., 

XLIX. 

SHuLL, G. H., 1911. Reversible Sex-mutants in Lychnis dioica. 

Bot. Gaz., LILI. 

SHULL, G. H.,1912. ‘‘Genotypes,” ‘‘ Biotypes,” ‘‘ Pure Lines,” and 
‘““Clones.”” Science, XXXYV. 

SHULL, G. H., 1912. Hermaphrodite Females in Lychnis dioica. 
Science, XXXVI. 

SmitH, G., 1910. Studies in the Experimental Analysis of Sex. 
ata and 2.9 Qed 1 S.. LIV: 

SmitH, G., 1910. Studies in the Experimental Analysis of Sex. 
Partss andi4: Ord OMS, LV, 

Smitu, G., 1911. Studies in the Experimental Analysis of Sex. 
parine (Qord 2 VM .. LVE 

SmitH, G., 1911. Studies in the Experimental Analysis of Sex. 
Parry Ore Oi aM S:, LVI. 

SmitH, G., 1911. Studies in the Experimental Analysis of Sex. 
Pater). aie So LVI 

SpEMANN, H., 1895. Zur Entwicklung des Strongylus paradoxus. 
Zool. Jahrb. Anat. Abt., VIII. 

SPILLMAN, W. J., 1908. Spurious Allelomorphism. Results of 
Recent Investigations. Am. Nat., XLII. 





274 BIBLIOGRAPHY 


SPILLMAN, W. J., 1909. Barring in Barred Plymouth Rocks. 
Poultry, V. 

SranpFuss, M., 1896. Handbuch der paliarktischen Grosschmet- 
terlinge. Jena. 

StranpFuss, M., 1910. Chaerocampa elpenor und einige Mittei- 
lungen itiber Aghia tau. Jris, XXIV. 

STAPLES-BRowngE, R., 1912. Second Report on the Inheritance of 
Colour in Pigeons, with Special Reference to Sex-limited In- 
heritance. Jour. Genet., II. 

STEINACH, E., 1912. Willkirliche Umwandlung von Sdaugetier- 
mannchen, etc. Arch. Gesell. Physiol., CXLIV. 

Stevens, N. M., 1905. Studies in Spermatogenesis with Especial 
Reference to the ‘Accessory Chromosome.” Carnegie Inst. 
Wash., pub. 36. 

Stevens, N. M., 1906. Studies in Spermatogenesis. II. Carnegie 
Inst. Wash., pub. 36. 

STEVENS, N. M., 1907. Color Inheritance and Sex Inheritance 
in Certain Aphids. Science, XXVI. 

STEVENS, N. M., 1908. The Chromosomes in Diabrotica, ete. 
Jour. Exp. Zodl., V. 

Stevens, N. M., 1908. A Study of the Germ-cells of Certain 
Diptera. Jour. Exp. Zoél., V. 

Stevens, N. M., 1909. Further Studies on the Chromosomes of 
the Coleoptera. Jour. Exp. Zoél., V1. 

Stevens, N. M., 1909. An Unpaired Chromosome in the Aphids. 
Jour. Exp. Zoél., VI. 

StevENS, N. M., 1910. A Note on Reduction in the Maturation of 
Male Eggs in Aphis. Biol. Bull., XVIII. 

Stevens, N. M., 1910. The Chromosomes in the Germ-cells of 
Culex. Jour. Exp. Zool., VIII. 

StEvENS, N. M., 1910. An Unequal Pair of Heterochromosomes 
in Forficula. Jour. Exp. Zoél., VIII. 

Stpvens, N. M., 1911. Further Studies on Heterochromosomes in 
Mosquitoes. Biol. Bull., XX. 

STEVENS, N. M., 1911. Preliminary Note on Heterochromosomes 
in the Guinea Pig. Biol. Bull., XX. 

Srevens, N. M., 1911. Heterochromosomes in the Guinea Pig. 
Biol. Bull., X XI. 


BIBLIOGRAPHY 279 


SrockKarpD, C. R., 1908. Habits, Reactions and Mating Instincts 
of the “Walking Stick,’ Aplopus mayeri. Papers Tortugas 
Lab., II. 

STRASBURGER, E., 1910. Ueber geschlechtbestimmende Ursachen. 
Jahrb. wiss. Bot., XLVIII. 

Strona, R. M., 1912. Another View of Sex-limited Inheritance. 
Science, XXXVI. 

Srrona, R. M., 1912. Results of Hybridizing Ring-Doves, In- 
cluding Sex-Linked Inheritance. Biol. Bull., XXIII. 

STURTEVANT, A. H., 1911. Another Sex-limited Character in 
Fowls. Science, XX XIII. 

StuRTEVANT, A. H., 1912. An Experiment Dealing with Sex- 
linkage in Fowls. Jour. Exp. Zool., XII. 

STURTEVANT, A. H., 1912. Is there Association between the Yellow 

and Agouti Factorsin Mice? Am. Nat., XLVI. 

STURTEVANT, A. H., 1912. Federley’s Breeding Experiments with 

the Moth Pygera. Am. Nat., XLVI. 

STURTEVANT, A. H., 1918. The Linear Arrangement of Six Sex- 
linked Factors in Drosophila, as shown by their Mode of 
Association. Jour. Exp. Zoél., XIV. 

SuRFACcE, F. M., 1912. The Diagnosis of Infectious Abortion in 
Cattle. Ann. Rep. Kent. Agr. Exp. Sta. 

Sutton, W. S., 1902. On the Morphology of the Chromosome 
Group in Brachystola magna. Biol. Bull., IV. 

Surron, W.S8., 1903. The Chromosomes in Heredity. Biol. Bull., 
IV. 

TANNREUTHER, W.S., 1907. History of the Germ-cells and Early 
Embryology of Certain Aphids. Zool. Jahrb. Anat. Abt., XXIV. 

TENNENT, D. H., 1911. A Heterochromosome of Male Origin in 
Echinoids. Biol. Bull., XXI. 

TENNENT, D. H., 1912. Studies in Cytology, I and II. Jour. 
Exp. Zool., XII. 

TuHomsEN, E., 1911. Die Differenzierung des Geschlechts und das 
Verhiltnis der Geschlechter beim Hiihnchen. Arch. Entw.- 
mech. der Org., XXXI. 

Tower, W. L., 1906. An Investigation of Evolution in Chryso- 
melid Beetles of the Genus Leptinotarsa. Carnegie Inst. 
Wash., pub. 48. 





276 BIBLIOGRAPHY 


Toyama, K., 1907. Studies on Hybridology of Insects. I. Bull. 
Col. Agr. Tokyo Imp. Uni., VII. 

Toyama, K., 1912. On Certain Characteristics of the Silkworm 
apparently Non-mendelian. Biol. Cent., XXXII. 

Tretsakorr, D., 1905. Die Spermatogenese bei Ascaris megalo- 
cephala. Arch. mikr. Anat., LXV. 

Trow, A. H., 1918. On the Inheritance of Certain Characters in 
the Common Groundsel— Senecio vulgaris— and its Segre- 
gates. Jour. Genet., II. 

Trow, A. H., 1913. Forms of Reduplication— Primary and 
Secondary. Jour. Genet., II. 

v. TSCHERMAK, A., 1912. Ueber Veranderung der Form, Farbe 
und Zeichnung von Kanarieneiern durch Bastardierung. 
Arch. f. Gesell. Phys., CXLVIII. 

v. TscHERMAK, E., 1908. Der moderne Stand des Vererbungs- 
problems. Arch. Rass. und Gesell., V. 

DE VILMORIN, P., and W. Batsson, 1911. A Case of Gametic 
Coupling in Pisum. Proc. Roy. Soc., B, LX XXIV. 

DE Vrigs, H., 1901. Die Mutationstheorie. Leipzig. 

pe Vrigs, H., 1908. Uber die Zwillingsbastarde von Cnothera 
nanella. Ber. deutsch. Bot. Gesell., X XVI a. 

DE Vrigs, H., 1908. Bastarde von CEnothera gigas. Ber. deutsch. 
Bot. Gesell., X XVI a. 

DE VriEs, Huao, 1909. On Triple Hybrids. Bot. Gaz., XLIX. 

pe Vriss, H., 1911. Uber doppeltreziproke Bastarde von (no- 
thera biennis und O. muricata. Biol. Centr., XXXI. 

WaLkKer, C. E., 1908. The Influence of the Testis upon the 
Secondary Sexual Characters of Fowls. Proc. Roy. Soc. Med., 
1908. 

WENKEE, K., 1906. Anatomie eines Argynnis paphia — Zwitters. 
Zeits. f. wiss. Zool., LX XXIV. 

WHEELER, W. M., 1903. The Origin of Female and Worker 
Ants from the Eggs of Parthenogenetic Workers. Science, 
AV LET 

WHEELER, W. M., 1910. The Effects of Parasitic and other Kinds 
of Castration in Insects. Jowr. Exp. Zool., VIII. 

Wuee ter, W. M., 1910. A Gynandromorphous Mutillid. Psyche, 
XVII. 


BIBLIOGRAPHY OTT 


Wuirtney, D. D., 1912. Reinvigoration produced by Cross Fer- 
tilization in Hydatina senta. Jour. Exp. Zool., XII. 

Wivper, H. H., 1904. Duplicate Twins and Double Monsters. 
Am. Jour. Anat., ITI. 

Wiuuiams, L. W., 1907. The Significance of the Grasping Antenne 
of the Harpactecoid Copepods. Science, XXV. 

Witson, E. B., 1899. The Cell in Development and Inheritance. 
New York. 

Wuson, E. B., 1905. Studies on Chromosomes, I and II. Jour. 
Exp. Zodl., I. 

Witson, E. B., 1906. Studies on Chromosomes, III. Jour. Exp. 
Zool., IIT. 

Witson, E. B., 1909. Studies on Chromosomes, IV and V. Jour. 
Exp. Zool., VI. 

Witson, E. B., 1909. Recent Researches on the Determination and 
Inheritance of Sex. Science, X XIX. 

Witson, E. B., 1909. Secondary Chromosome Couplings and the 
Sexual Relations in Abraxas. Science, XXIX. 

Witson, E. B., 1910. Note on the Chromosomes of Nezara. 
Science, XX XI. 

Witson, E. B., 1910. The Chromosomes in Relation to the Deter- 
mination of Sex. Sci. Progr., XVI. 

Witson, E. B., 1910. Studies on Chromosomes, VI. Jour. Exp. 
Zool., IX. 

_Witson, E. B., 1911. Studies on Chromosomes, VII. Jour. 
Morph., XXII. . 

Witson, E. B., 1911. The Sex Chromosomes. Arch. Mikr. Anat., 
LXXVII. 

Witson, E. B., 1912. Studies on Chromosomes, VIII. Jour. 
Exp. Zool., XIII. 

Witson, E. B., 1912. Some Aspects of Cytology in Relation to the 
Study of Genetics. Am. Nat., XLVI. 

Wourereck, R., 1911. Uber Verdinderung der Sexualitit bei 
Daphniden. Leipzig, 1911. 

Woop, J. G., 1874. Insects Abroad. New York. 

Wooprurr, L. L., 1905. An Experimental Study of the 
Life-History of Hypotrichous Infusoria. Jour. Exp. Zodl., 
) Wh 





278 BIBLIOGRAPHY 


Wooprurr, L. L., 1908. The Life Cycle of Paramecium Proc. 
Soc. Exp. Biol. and Med., 1908. 

Wooprurr, L. L., 1908. The Life Cycle of Paramecium when 
subjected to a Varied Environment. Amer. Nat., XLII. 
Wooprurr, L. L., 1908. Effects of Alcohol on the Life Cycle 

of Infusoria. Biol. Bull., XV. 

Wooprurr, L. L., 1912. A Summary of the Results of Certain 
Physiological Studies on a Pedigreed Race of Paramecium. . 
Bioch. Bull., I. 

Woopkrurr, L. L., 1912. Observations on the Origin and Sequence 
of the Protozoan Fauna of Hay Infusions. Jour. Exp. Zodl., 
XII. 

Wooprurr, L. L., and G. A. BartseLut, 1911. Rhythms in the 
Reproductive Activity of Infusoria. Jour. Exp. Zoél., I. 
ZACHARIAS, O., 1912. Zur Cytologie des Kies yon Ascaris megalo- 

cephala. Anat. Anz., XLII. 

ZimcuER, H. E., 1911. Die Chromosomen als Vererbungstrager. 

Jahresb. d. Vereins. vaterl. Naturk. Wiireburg, LX VIL. 


INDEX 


Abraxas, 128 
Achates, 151 
Achia, 106 
Addison’s disease, 147 
Adkins, 217-218 
Adrenal, 147 
Agenor, 151 
Allen, 113 
Amphibia, 145 
Amphipoda, 117 
Andrews, 117 
Angiostomum, 170 
Antlers, 110, 133 
Ants, 117 
Argentine, 227 
Argonauta, 26 
Aristotle, 35 
Armadillo, 238 
Ascaris, 20, 21, 49 
Ascidian, 217 


Baltzer, 55, 58, 61 
Bancroft, 194 
Barnacle, 155 
Bateson, 72, 75, 99, 100, 125 
Baur, E., 99 
Beans, 123 
Bee, 174, 175, 176, 220 
Bees, 32 
Beetles, 106 
Bell, 232 
Belt, 102 
Bird of paradise, king, 109 
six-shafted, 109 
superb, 109 
Black, 96-97 
Blakeslee, 171 
Bobolink, 27 


Boring, 51 
Bovyeriol,- oo, O8,. 162, 165, 170, 
171 


Bresca, 145 
Bridges, 223, 224 


Bruce, 212 
Bryonia, 171-172 
Bitschli, 8 


Calkins, 8, 198, 206, 209, 210 
Callosamia, 116 
Capons, 142, 1438 
Cardamine, 215-216 
Castle, 195 

Ceylon, 125, 127 
Checker diagram, 78 
Chemotaxis, 117 
Chidester, 117 
Cicada, 106 

Ciona, 217-218 
Clipped wings, 119 
Colas, 129-130, 150 
Collins, 202 
Color-blind, 241 
Color blindness, 242 
Conger eel, 2 
Corpus luteum, 147 
Correns, 74, 79, 99, 171, 172, 215, 216 
Crab, 155 
Cretinism, 146 
Cricket, 150 
Criodrilus, 168 
Cuenots 232.230 
Cunningham, 121 


Daphnians, 182-185, 189 

Darwin, C., 73-74, 101, 103, 104, 107, 
112-114, 120, 125, 142, 194, 197, 
200-202 

Davenport, C., 72, 143, 239 

Deer, 110, 133, 134 

Delage, 193 

Dinophilus, 234 

Diplogaster, 225 

Doncaster, 176 

Dorsets, 134, 135, 186, 137, 188 

Drelincourt, 232 

Drone, 175 


279 


280 


Drosophila, 63-68, 96, 117, 130 
Diising, 233 


East, 99, 202, 204, 211 
Edwards, 51 

Egret, 111 

Eland, 136 
Elaphomyia, 106 
Elephant, 110 
Emerson, 99 

Eosin eye, 130, 154, 155 
Eupaguras, 158 
Euschistus, 151. 


Habre. 220, 221 
Fielde, 117 

Firefly, 28, 30, 31 
Fishes, 32 

Florisuga, 102 

Foot, 151 

Forel, 117 

Frog, 145, 147, 228 
Frolowa, 51 

Fruit fly, 117, 195, 196, 221 
Fundulus, 32 

Gall, 179 

Galton, 236 

Game, 144, 212 
Geddes, 232 

Gentry, 232 
Germ-cells, 23 
Gerould, 130, 150, 151 
Giard, 155 
Gigantism, 146 
Goldschmidt, 124 
Goodale, 72, 142 
Gosse, 103 

Growth, 3 
Gudernatsch, 147 
Guinea hen, 225 
Gulick, 51 

Guyer, 225-226, 245 
Gynandromorphism, 161 
Gypsy moths, 117 


Habrocestum, 107 
Hzemophilia, 239, 240, 242 
Hectocotylized arm, 26 
Henking, 50 

Herbst, 55, 61, 62 
Herdwicks, 134-135 


INDEX 


Hermaphroditism, 161 
Hertwig, R., 9, 228, 234 
Holmes, 117 

Hormones, 146 

Horns, 133-138 

Hudson, 114, 115 
Humming-birds, 103, 108 
Hydatina, 2, 185 

Hyde, 196, 199, 215 


Ichthyosis, 242 
Identical twins, 236-239 
Inachus, 155 

Ipomecea, 197 

Italian, 227 


Jacobson, 151 

Janda, 168 

Janssens, 94 

Jehring, 238 

Jennings, 9, 12, 206-208 
Johannsen, 122-125 
Jordan, H. E., 245 


Keeble, 212 

Kellogg, 117, 232 
King, 229, 234 
Kopec, 149 

Kruger, 225 
Kuschakewitsch, 228 


Lamarckian school, 17 
Landois, 232 
Langshan, 69-71 
Laomedon, 151 
Lethal factor, 221-223 
Linkage, 93 

Lion, 27 

Lister, 34 

Loeb, J., 62, 190, 191, 192, 193 
Lutz, 118 

Lychnis, 172-173 
Lygzeus, 44 
Lymantria, 148 


McClung, 50 

Meevia, 108 

Mallard, 28, 142 

Malsen, von, 234, 235, 236 
Mammals, 159 

Mammary glands, 140 
Man, 34, 229, 236-249 





INDEX 


Marchals, 171 

Mast, 30 

Maupas, 5, 8, 187, 198, 234 
Mayer, 116 

de Meijere, 151 
Meisenheimer, 145, 148-149 
Mendel, 84, 73-75, 80, 84 
Menge, 34 

Merino, 134, 135 

Miastor, 21, 174 

Mice, 233 

Mimicry, 127-130 
Miniature wings, 66—67 
Mirabilis, 79-80 
Moenkhaus, 196 
Montgomery, 34, 50, 115, 117, 245 
Mosquito, 51 

Mosses, 171 

Mulsow, 51 

Myopia, 242 


Nematode, 224-226 
Nereis, 36 

Neuroterus, 176-177 
Newmann, 238 

Night blindness, 242 
Non-disjunction, 223-224 
Nussbaum, 16, 145 





Ocneria, 148 

Octopus, 25 

Oncopeltus, 46, 84 

Optic nerve atrophy, 244 
Oudemans, 148 
Ovariotomy, 135 

Owl, 111 


Papanicolau, 183-185 
Papilio, 125-129, 151 
Parameecium, 5, 6, 12, 206-211 
Parathyroid, 146 
Parthenogenesis, 161 
Patterson, 239 

Paulmier, 50 

Pea, edible, 75-78, 85-88 
Pearl, R., 72, 212-213, 227 
Pearse, 117 

Peckham, 115-116, 120 
Pellew, 212 

Peltogaster, 158 
Petrunkewitsch, 117 
Phalarope, 112 





281 


Pheasants, 225 

Phidippus, 34 

Photinus, 28 

Phylloxerans, 52, 54, 178, 179, 180, 
181, 189 

Pigeons, 32 

Pituitary body, 146 

Plutei, 60 

Plymouth rock, 69-71, 212 

Polar bodies, 37 

Polytmus, 103 

Porter, 117 

Porthetria, 117, 148 

Primula, 201, 202 

Promethea, 116 

Protenor, 40 

Punnett,7127, 128; 138.233 


Rawls, 221 

Bat, 140. 233 
Reduplication, 100 
Reindeer, 136 

Rhabditis, 169, 224, 226 
Riley, 232 

Ritzema-Bos, 195 

Rotifers, 185-189 
Rudimentary wing, 214, 215 
Russo, 234 


Sacculina, 155 

Sagitta, 21, 22 

Schenk, 233 

Schleip, 170, 171 
Schultze, 233 

Sclerosis, 242 

Seabright, 143-144 

Sea cow, 27 

Sea-lion, Steller’s, 110 
Sea-urchin, 56-62 
Segregation, 81, 100 

Sex, 83, 84 

Sex chromosome, 50, 80, 83, 84 
Sex determination, 84 
Sex-limited, 83 
Sex-linked, 81, 83, 84, 132 
Sheep, 134-138 

Shull, A. F., 187, 197, 205 
Shull, G. H., 173, 202, 204, 211 
Shuster, 145 

Siamese twins, 236 
Silkworm, 117, 165 
Sinéty, 50 


282 


Skeleton, rat, 140 

Smith, G., 145, 155 

Soule, 116 

Sparrow, 2 
Spermatophores, 25 
Spheerechinus, 59-60 
Spiders, 34, 107, 115, 117 
Squid, 24 

Stag, 133 

Steinach, 140 
Stephanospheera, 5 

Stevens, 51 

Strobell, 151 
Strongylocentrotus, 59, 60, 62 
Sturtevant,./2, 98, 117, 118 
Stylonichia, 2 

Suffolks, 136-138 

Synapsis, 93 


Tadpoles, 147 
Tanager, scarlet, 27 
Thomson, 232 
Thymus, 146-147 
Thyroid, 146-147 
Toad, 229 

Tower, 117 
Toyama, 165 
Treat, 232 


INDEX 


Triton, 145 
Trow, 99 
Tschermak, 74 


Vermilion eye, 119 
Vestigial wing, 96-97 
Vigor, 120 

Vincent, 146 

de Vries, 74, 125 


Wallace, 102, 113-114, 120, 125, 127 

Wasp, 220 

Weismann, 16, 17, 40, 194, 195 

Wheeler, 117 

White eye, 62-65, 81, 82, 88-92, 118, 
119, 221-223 

Whitney, 185, 187, 197, 205 

Wilder, 237 

Wilson, 51 

Winiwarter, 245-248 

Wood, 136 

Woodruff, 8, 198 


X-chromosome, 51, 82, 84, 242 


Y-chromosome, 51, 84 
Yellow body color, 67, 88-92, 119 





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